These guys slogged through an incredibly grueling task - porting the entire .NET Framework and C# language - through some really hard times. I'm glad to see that it's paying off for them!
Well, yeah - but enough of that massive thing that it's a pretty impressive test of patience.
There's really no point to porting some parts.
I wonder how accurate this chart is. The last time we really wanted to port something from .net to mono (3 - 4 years ago) windows forms was supported, but it was a mess. It worked 90% - but the 10% never got fixed. Problems popped up in the strangest places (rounding of numbers). Impressive as a proof of concept, but we've stopped trying to support mono and never looked back.