TL;DR: it was a pejorative, then it was reclaimed, then it fell out of fashion, then it returned as a pejorative on internet message boards, except English speakers browsing the Japanese internet didn’t realize that was a pejorative and thought bara just meant hairy, muscled gay dudes. And now we’re here!
The history on why we call it bara in English is pretty wild: https://en.wikipedia.org/wiki/Bara_(genre)#Etymology
TL;DR: it was a pejorative, then it was reclaimed, then it fell out of fashion, then it returned as a pejorative on internet message boards, except English speakers browsing the Japanese internet didn’t realize that was a pejorative and thought bara just meant hairy, muscled gay dudes. And now we’re here!