(A) Because Quora in Spanish, French, German, and Italian already exist by now. There may even be more in the future. Maybe.
(B) Because Chris Tou’s answer to Does content on Quora need to be written in English?, from 2011, still holds:
However, there are probably still several good reasons to promote the use of one standardized on a site such as this. For example, using only one language allows everyone to be able to communicate and share. You won’t have someone giving an answer in, say, Chinese, and then have worry about translating it to another language for others to understand.
Another possible reason is that it’s hard to moderate posts in languages you do not understand. Quora relies on a form self-regulated community. Having separate languages promotes segregation and becomes hard for the community to self-regulate unless they spoke that specific language. On a forum where there are more members, that could work, as each specific language community would monitor itself, but Quora is not yet there, I think.
Not using English on Quora does exclude people that don’t speak that language; speaking English is one of the few prereqs to joining here.
So Questions and Answers not in English are verboten on Quora in English. It’s not as clear whether blogs can be not in English (Can I write a Quora blog in a language other than English?); reportedly they used to be explicitly allowed, with provisos of English topics and titles, and the blog जय महाराष्ट्र | Jay Maharashtra is still going strong. See discussion at https://www.quora.com/Can-I-writ…
I have seen comment chat between people in languages other than English, but very infrequently. I have occasionally tried to initiate that, in German and Greek; sometimes, it’s worked.