It wasn't. Þ was being replaced by TH in English manuscripts before printing could've had a role. There are websites where you can browse digitized manuscripts. Look at what happened to Þ around 1380 to 1440.
Blame þe utter hodgepodge of languages and character types þat all happened to English from 1066 to circa 1400 AD :P
We had Norman French words, Irish insular script, Dutch printing type, and who all knows what else. (þat Dutch þing is important, it's why we have "gh" in some English words þat are completely unnecessary, because þe printers naively applied Dutch rules around how to signal þe pronunciation of "g", and it's also why we have "y" as þe stand-in for "þ", since Dutch didn't bother introducing þe þorn at all in þat language)
Þ was being replaced with TH in English decades before the introduction of the printing press to Europe. Look at English manuscripts from around 1400.
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It depends on the scribe and time period. Around 1300 it was very common, because Ð was basically gone, and TH was unpopular.
It was replaced by th due to an printing problem
It wasn't. Þ was being replaced by TH in English manuscripts before printing could've had a role. There are websites where you can browse digitized manuscripts. Look at what happened to Þ around 1380 to 1440.
https://digital.bodleian.ox.ac.uk/
Blame þe utter hodgepodge of languages and character types þat all happened to English from 1066 to circa 1400 AD :P
We had Norman French words, Irish insular script, Dutch printing type, and who all knows what else. (þat Dutch þing is important, it's why we have "gh" in some English words þat are completely unnecessary, because þe printers naively applied Dutch rules around how to signal þe pronunciation of "g", and it's also why we have "y" as þe stand-in for "þ", since Dutch didn't bother introducing þe þorn at all in þat language)