A lot of people who like *nix are precluded from it, without some sort of major workflow change. After getting excited about OSS languages, it's hard to just dump that and go full-stack Microsoft. Hell, a place I just interviewed at literally has everything through Microsoft that's feasible. IIS to the damn mice. And SourceSafe.
I do agree, the OSS stack for .Net is not as nice as Microsoft's, but in my case I mostly do everything with Emacs anyways, so it doesn't bother me. MonoDevelop is not bad though.
Unofficial, but actually pretty decent (if you are into IDEs) is MonoDevelop.