I can't give a definitive answer, but it makes sense in the context of the Russian and Chinese Empires. In America, all minorities are fundamentally American(or soon to be) and the "racial" distinction is entirely pointless. Might be some cultural differences depending on region/location but they aren't as massive as people think.

Whereas in the Russian Empire, Kazakhs, Ukranians, Mongols, Volga Germans etc etc all had different languages, religions, and cultures and were under a prominently Russian Empire. The USSR was founded in the back end of the Age of Nationalism and various things were done because of this. Giving the various non-Russian regions varying autonomy allowed them to basically not "Russify" and thus not make the USSR just another "Russian Empire" but a "confederation" of various republics. Not perfect, but the general jist. I imagine China was very similar.

The closest thing the USA(and Canada) has is the Native Americans but they were heavily wiped out over centuries so they aren't as numerous as the other 2.

You can find various ideological justifications but in the end it comes down to pragmatism and having bigger problems to solve. Restless minorities can often be placated by giving them various degrees of autonomy, and for a state beset by hostility on all sides and dealing with a very precarious economic, military, and political situation, sometimes letting minority X run its own affairs while you tackle inheriting one of the two most massive, backwards, and undeveloped states on the planet (Tsarist Russia and Qing China) after a century of apocalyptic war, is just intelligently picking your battles.

From my understanding, China is far less ideological than Western media often portrays. Rather than being driven by grand ideals, it tends to take a pragmatic approach: granting limited rights and autonomy to significant minority groups to keep stability and govern at the lowest possible cost, while concentrating development and resources on the eastern coastal core.

Culturally, China is different from the West. It doesn’t have the kind of religious or missionary zeal to convert, eradicate, or forcibly remake “barbarians” into copies of the majority. Even though Marxism claims certain universal principles, it doesn’t override millennia of imperial governance traditions and cultural habits overnight.

Historically, assimilation in East Asia has been slow and incremental, and the PRC largely follows this pattern. As a result, many traditional cultures have been preserved far better on the Chinese side than they were under the Soviet system

Probably for internal stability. It works for them because they’re not immigrant countries.

Imagine trying to give every ethnic minority their own autonomous region. It’s just impractical. China only officially recognizes 52 ethnic minorities. There’re probably other groups that aren’t recognized.

For example, China considers Hui people, Han Chinese Muslims, as an ethnic minority, but Han Chinese Christians aren’t considered an ethnic minority due to I assume an insignificant population size.

The real answer is because these policies were drafted in the early 20th century when Nationalism was both assumed to be the political default as well as being the most progressive way to organize society.

And as for afterwards, ethnic reorganization is not exactly something you can take back so the divisions remain.

There are plenty of Russians who regret allowing the Baltics to have their own ethnostates (before the Soviets redistributed their populations, actual Latvians and Lithuanians were minorities in the territories that make up the current countries).

Stalin’s book on the national question provides a good view into how they conceived of the issue. Remember that large parts of what eventually became the USSR were not Russian as much as were swallowed by the Russian empire. China was a similar story. 

I get your point of view, but it implies a certain equality at the starting line that does not exist in our world. The giving certain privileges to certain “nations” (in quotes because both literal nations and unofficial nations like native tribes apply) is an acknowledgment of the backwardness that was often thrust upon them by history. 

While not directly related, I think some of the arguments that created the foundations of Reconstruction in the US would be interesting reading to you. 

A lot of people here are giving you very pragmatic explanations, and while I’m sure there’s some truth to it(i.e. groups of people are more likely to go along with your grander vision if you carve them a little autonomy), when you read what these people wrote, it’s more than just a calculated pragmatism behind these decisions. 

Its something any country, communist or capitalist, will try if they have a tonne of ethnic minorities-especially close to the fringes of your territory/control, where you suspect that using the stick far more than the carrot will yield worse results. Attempting to reconcile minorities to you involves some mixture of punishing opposition and making compliance appealing-hence autonomy and special rights. Your "own" people are held to sometimes need less carrot because they're your people, they have their own nation, and that counts for something because we have been living in the age of nations. It does make people a little more passive and a little less resentful if they are at the core of their nation and they can't construe it as somehow being a foreign imposition.

In the case of native americans in North America, both the US and Canada, their numbers been whittled down to the point that any bone you throw them is very cheap and easy by virtue of the fact that there are very few of them, however construed, to avail of it. Very little leverage for the native americans combines with very little cost for the state to make concessions such that they still get some token ones.

Why in the same nation do different racial groups have different rights?

This comes from Lenin. He distinguished between the nationalism of oppressor nations and the nationalism of oppressed nations.

https://www.marxists.org/archive/lenin/works/1922/dec/testamnt/autonomy.htm

...must consist not only in the observance of the formal equality of nations but even in an inequality of the oppressor nation, the great nation, that must make up for the inequality which obtains in actual practice. ...

In one way or another, by one's attitude or by concessions, it is necessary to compensate the non-Russian for the lack of trust, for the suspicion and the insults to which the government of the "dominant" nation subjected them in the past.

This stems from the reality that, in every sense, ethnic minority regions did not automatically belong to the communist revolutionary party. They had their own anti-colonial revolution, their own communists, etc.

Lenin worked in the Volga region and had firsthand experience of how the Tatars and other groups harbored resentment due to their treatment by the Russians. As a communist, how will you convince these ethnic minorities that you are different from the imperialists and are not trying to impose your ethnic group's agenda on them? This is Lenin's idea. Only by correcting the hierarchical structure within the workers can this division be healed.

In form, early USSR and China did indeed develop according to his vision, but in substance, not so much.

A major part people are missing is that China and the USSR both believe (USSR did, China still does) at a political level and scientific level that Biological Race is a real concept based in biological materialism.

I think historically it emerges out of discussions in the Austrian Hungarian about various repressed ethnic groups that want to become nations. Stalin"s probably most significant theoretical work is about ethnic groups.

I thought one of the big differences between China and the USSR was that China did not have ethnic republics?

I know a little bit about Vietnam and the "nationality" question is a difficult one. As you point out there is a very weird fabrication of ethnicity and the solidification of groups that probably didn't really exist before classification. They also are really "indigenous" with some only having been in Vietnam (or what would become Vietnam) for a couple of hundred years.

Vietnam has 54 ethnic groups based on Stalinist principles, and probably some Chinese/French race science (when they created the schema the government was at war with China) It is particularly attached to the number (it four or five less then the Chinese) and has gone out and of it's it way to preserve the number on paper. Outside of the Hoa(Chinese) they are largely rural and poor. As far as I can tell there there is a type of affirmative action program in which they get extra points and there is an attempt to create an ethnic elite via schools + preservation of a certain culture based on ideas of a national literature etc. Honestly a lot of It sounds like an attempt to create a local elite to govern a region that It would be impossible to govern otherwise. one of the few groups in Vietnam that have seemed limited progress in terms of wealth under Doi Moi, so I feel there is a big global NGO element as well.

There was a semi autonomous zone established in the north west after the 1st Indochina war but it sounds like it was mostly for show and an attempt to prevent the whole region blowing up. It's hard to find out out but it sounds like certain groups were seen as feudal overlords- apparently the Tai were targeted in particular.

There seems to be a significant civilising/securitization mission attached to this. All the nomadic groups were settled and there is a weird civilisation element- the Vietnamese create a written language for all the official group.

Some ethnic minorities worked heavily with US special forces and then the Cambodian government of the 60s/70s. The story is really really unclear but officially some kind of struggle continues until the early 90s but who knows if it was just on paper.

ATM there seems to significant repression in relation to religion particularly conversion to Christianity (which is connected to the war/American evangelism), repression of millinialisn parrticularly among the Hmong, land grabbing and internal security fears such as FULRO (this is probably largely bullshit)

So I guess the answer is that I see it as product of "progressive" state building and the need to create a local elite to rule. I guess that isn't a good theoretical answer.