Why C# doesn't have first class functions and can't go fully functional (and likely never will): Scala tried and its compiler is slow, even after so many iterations and new novel compiler ideas.
I like the way it is, and hope it doesn't change. Unless they could make this possible without making the compile process extremely slow.
I love C# and .NET and I use them extensively but I dislike the fact that the framework still kind of forces you into OOP.
If you like to have a more functional workflow you have to roll your own stuff or wrap objects in functional constructs.
But .NET is excellent for startups because:
-it is very fast to develop in
-has excellent tooling
-it is batteries included
-generally you have ONE way to do things that is accepted by most developers
-documentation is very good
-it supports large codebases with ease
-community is large enough
-you can use it for many areas, from embedded, to high performance computing, to desktop, mobile, web backend and web frontend
-it is performant