Because we'd love to use C# across more platforms, and the 2nd class citizen status of Mono in the *nix world makes us sad :(
Ah just so they have access to the vast resources and become 1st party libraries. Mono is not known in most of the linux world and Microsoft support would be a significant boost. Businesses would also be hesitant to support it adopt it unless Microsoft is backing the platform.
Microsoft supporting Mono would not be a good boost in the eyes of most Linux people, as far as I can tell. In fact, Miguel de Icaza connections to Microsoft have been cited as an argument against Mono.
The fact that Mono is a reimplementation of something Microsoft did actually went against it for a lot of *nix people. Which is dumb and stupid, but hey, after Embrace Extend Extinguish, everyone got kinda jumpy. So, Microsoft purchasing it would be a hard sell, IMO.