There's still a ton of C legacy code running on windows machines out there.
Forcing legacy code to stay woefully out of date to support a single vendor's stubbornness, perpetually growing its maintenance burden as the rest of the world moves on. Hooray for the Microsoft way!
C90 isn't going away, and it's not Microsoft's fault. Many C compilers still support 1st edition K&R style for a reason — old C code is going to stick around, and it's probably not going to change much, let alone in any way that needs C99 features.
Ad 2) If there is support for features there will be people using them.