Short answer: I don't know.

Long answer: As far as I've been able to tell, InDesign only creates separate groupings for Arabic, Hebrew, Thai, Chinese, Japanese, and Korean. Everything else simply gets grouped together with Latin. I suspect this may have to do with the fact that the menu was designed with Adobe fonts in mind, and that the menu architecture was finalized sometime after the release of Adobe Thai but before the release of their more recent Indic fonts. Presumably Greek and Cyrillic don't get separate menu ranges since all adobe fonts which cover these languages also cover latin.

André

I am pretty sure Andre's explanation is right.

I was involved in vetting the heuristics back in the day, with Sairus Patel who was creating them.

In the end, I came to the conclusion that any such effort to simply segregate the font menu into static sections based on language support was doomed to create more problems than it solved, but I was told the client apps insisted on it. So here we are.

Allowing the user to filter by support for some particular writing system or language would be cool, though. But a lot more work to implement the interface.