IMHO, it's the "open" part and less the "source" part. I believe a lot of projects were reluctant to jump on the Mono train because they expected it would be steamrolled (on purpose or just through sheer manpower) by the .NET. However, if everyone is building on top of the same CoreCLR, it's possible that fear might subside. I doubt it will ever go away because Microsoft will continue to keep some APIs close to the vest.

If it's possible to transpile either Java source or better Java class files to a CLR assembly, even some of the community momentum that Java has might turn into a benefit for .NET, too.