The the larger problem is one of self-selection. C# devs are not alone in not contributing to opens source software. The vast majority of devs don't make meaningful contributions to OSS. It's just the the few people who get really stoked about open source software and love making a positive contribution to world through software have been turned off by Microsoft's behavior for the last 10-15 years. So they never entered the .NET/C# world to begin with.
The second is incentives. Java shops are much more comfortable with open source then .NET shops. Open source is still terrifying to many enterprise customers, and a lot of .NET shops. And the incentives besides making a positive contribution to the world [0] for contributing to OSS are it looks good on your resume, maybe you could consult on it, and getting to be paid to work on your own OSS project. But with most customers not using OSS the developers don't get a lot of personal benefits from it.
The third is how bug fixing your favorite OSS acts like a gateway drugs. In the past .NET devs wouldn't have very much experience with any OSS. If they found a bug in their favorite framework they would report it. End of story. Everything they used was closed source MS stack. But if they were working with Java(and probably working on an OSS library) they could dive into that library and make the fix themselves. Now that all of the core libraries for MS are being open sourced many more devs will start to get familiar with what it's like to be able to make a change to library or framework you use on a daily basis. And hopefully some will catch the OSS bug.
I think this will change. As Microsoft pushes more OSS, enterprise customers will see it as less scary. Devs who are into OSS might start considering .Net as an option. And finally devs will start to get a taste of what it's like to get to fix a bug on a framework or library that 1000's of people use.
[0] -see point 1 why people who wanna make a positive contribution to world through open source software shy away from being .net developers