It's hard to know for absolutely certain, but a lot of it might be because diacritics would just make English even harder to read/write. By the invention of pri

It's hard to know for absolutely certain, but a lot of it might be because diacritics would just make English even harder to read/write. By the invention of printing, English was a very confusing mess of the germanic and romantic languages and there was no absolute agreement on pronunciation. Plus, we were in the middle of the great vowel shift, so slapping diacritics on letters would have been a fools errand since they wouldn't sound like that diacritic says they do. English's lax pronunciation, linguistic changes during the middle ages, and strong romantic language influence, would make it even more confusing to create solid rules on diacritic usage, especially as many people would still be illiterate, and English was an "easier" language than the very complex Latin.