Microsoft is still the one leading .NET, defining what .NET is and what technologies it includes (and which it doesn't). While Mono is a nice subset that runs on non-Microsoft OSs, .NET is pretty much a Microsoft-centric technology.
You don't need to go further than the recent announcements and the excitement they generated among .NET users to see how dependent on Microsoft they are. It's Microsoft's show to run.
Technically, Windows is no longer a windows technology, since Wine exists, but still it's mostly windows. Can you even run Visual Studio on another platform?
Visual Studio is still pretty dependent on Win32 / x86 under the hood - a lot of the UI was ported to WPF and more of the tooling, including the compiler, is being ported to C#.
So no, I think VS for Mac / Linux is still a ways off. They're moving in that direction though - wouldn't be surprised if they bought Xamarin in order to make it happen.
You don't need to go further than the recent announcements and the excitement they generated among .NET users to see how dependent on Microsoft they are. It's Microsoft's show to run.