/r/linguistics would be glad to answer this question

hi! you may be interested in responses to these earlier related questions

When did writers of English stop capitalizing all their nouns? And why?

Why were words capitalized in the middle of sentences in various 18th century US documents, such as the Declaration of Independence, and why did 19th century documents put periods in titles?

Late eighteenth century. Basically it was the introduction to the English world of italics for emphasis that became an aesthetic thing popular with the intellectuals, after Alexander Pope and some other tastemakers really thought Italian books were sophisticated. After you have italics, you don't need the Capitalization For Emphasis That Was Popular In Ye Olde Tymes and English language presentation starts to drift away from the Fraktur/Gothic Germanic blackletter stuff