Its not that it isn't allowed. If you feel comfortable in typing in the Devnagiri script, you are free to do it.
But the problem is we have English keyboards and users translate it English to Hindi through translators which causes spelling mistakes and grammatical errors. Which is why most of them type in English.
However, everyone is free to use to both English and Hindi (Devnagiri script) here. But keep in mind that no other languages are allowed here apart from these two.