The comments section of this blog post illustrates the problems I have with the .NET "community", as a "part-time" .NET programmer.
It's full of "what a waste of time!" & "why would anyone use this?" comments. It illustrates the lack of vision and inventiveness in the community. I guess "because I could", or "because I wanted to(.|learn)" wasn't good enough.
Aside from a small minority of brilliant folks like Hanselman, the .NET community is pretty bleak, boring, and stale. It's hard for me to get excited about anything in that world when I know they're just cribbing features that have been around the OSS world for a few years (I'm looking at you ASP.NET MVC).
Why work with engineered ripoffs, when I can just use the real thing for free (and without any ridiculous licensing)? The real thing (Rails/Django/PHP/etc) is more organic and agile, oh and it's freakin' FREE!
Not saying there isn't room for a lightweight C# framework though; I don't know of anything beyond ASP.NET.
They don't, generally. C#/.NET hasn't exactly been a fertile breeding ground for innovative new open source projects. The most important .NET project are typically Java or Ruby ports (unit testing ports, Rails-alikes, ORM ports, NAnt, etc.).