It is mostly because of the ecosystem, due of the proprietary nature of the .NET Framework not many are willing to put in their personal time to build libraries

It is mostly because of the ecosystem, due of the proprietary nature of the .NET Framework not many are willing to put in their personal time to build libraries and other supporting software around it which results in the lack of any real community. Most startups today are well connected with some community or other and to them .NET is mostly invisible. On the other hand if a person from an enterprise builds a startup and if they have a .NET background that is when they try to get into bizspark.