Nothing except that it wasn't a priority for them. Maybe it is now.

I'm pretty sure releasing a version of the .NET framework and runtime that works across platform is a little more difficult than changing the target architecture on their compiler settings (does their compiler even support different architectures?)

I'm not saying it isn't possible - just that it wasn't a priority for them. This doesn't mean they don't want or support a cross-platform .NET ecosystem. (If I recall something Miguel De Icaza wrote some time ago, the internal teams in Microsoft have been supportive of Mono.)

Do agree though that if this were an aquihire or defensive move, then it sucks for the rest of us.