As meanguy pointed out, the Mono team lost Novell's backing -- arguably, they never had it in the first place. Xamarin is now focused entirely on mobile, to the detriment of the core CLR implementation and web.

Not that I blame them in the least. I've been a huge fan of Miguel for years, and they're doing great things in the mobile space. I just can't in good conscience invest heavily in Mono knowing that it's essentially at a dead end -- particularly when other much more attractive web technologies have been released since .NET's inception.

The reality is that Microsoft never really wanted to build a cross-platform CLR. They wanted a great Java-like runtime that only works on Windows. If that matches up with your goals, then by all means use .NET, but be prepared for a tough slog later on if you want to escape Windows.