Perhaps the user demand is not so big or even noticeable? It starts to make sense when you type in more than two languages regularly. I would argue that there are only couple of percents of bilingual users and people using three and more languages are even less common.
You have to switch to Latin to type things like emails, URLs, foreign band names etc. And if you are writing a code then you have to switch constantly to type comments and messages in your naive language. So inconvenient.
Bilingual users are not that rare, at least at school most people in my country study some foreign language. Of course it doesn't mean they are going to use it after graduation, but anyway.