They never got the full stack running on the server and they punted most of the Windows-specific client stuff from the start.
They landed on a super smart subset and seem to be kicking ass with it. A C# compiler with some odd omissions and cool enhancements + native bindings to iOS and Android equals a damn useful tool. If you're building .NET or even Java backends it's certainly a very sane way to hook into them from Android phones and tablets in the enterprise.
But it's not a cross-platform .NET environment by any stretch and certainly isn't on the path to becoming one.