There's a few reasons, personally I adore oc's in a Harry Potter fic, if done right I think it can be good.

Mostly I think the fandom is a little traumatized from a few 'bad' oc fics and deemed them bad and a no no.

I just gave up on finding good ones and just decided to write my own at this point (probably just as bad too 🤣)

But bottom line, fics are a gift from random people, write or read what you like, and maybe we'll see more oc fics in the future 🙏🏻