You already have people writing in languages that have huge outreach and audience willing to read in that language, i.e. Mandarin.
But English is the modern lingua franca, and most of the Internet functions in English. The sooner one masters it, the higher are their chances to be succesful on the Internet. We are already working to capture niche audience. Cutting that niche audience even smaller by hoping that there are people among those who speak your language AND are interested in your niche is setting yourself up for a smaller audience to begin with.
Reading a language is usually easier than writing in a language, so you immediately widen your audience when you switch to English, including adressing youself to huge populations of the countries that are multilingual but use English as the second language. English is almost globally taught in schools as the first pick of a foreign language.
Even on this site I saw stories written in Spanish (a big language) whose authors cheated so that their stories ended up on top of the popular section in romance, and nobody was viewing them outsides the bots they set up to rake up the subs and likes.
Writing takes a lot of time and efforts, so writing something in a language other than English or doing the double lifting with translating (which is an almost lost art in itself in our age of the Google translator apps), in a vague hope that other people would do so as well, and THEN they will come is pretty harsh proposition.
Particularly if you are an immigrant, so you have zero access to the publishing industry in your country of origins, so you don't even have that hope warming you up.
