Probably because it isn't an official port, there is no reason to learn .NET if you don't develop exclusively for Microsoft.
If you're developing for multiple platforms chances are you already use some other toolkit with platform specific bits mixed in, mono is only useful if you are a .NET developer and then decide you want to port apps.
If you're developing for multiple platforms chances are you already use some other toolkit with platform specific bits mixed in, mono is only useful if you are a .NET developer and then decide you want to port apps.