Nothing stopped Microsoft from releasing an open-source .NET implementation that works cross-platform
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.
The C# compiler targets the CLR (a virtual machine), so doesn't need to be re-targeted. They had/have commercial versions of the CLR for ARM (.NET Compact Framework), x86, x64 and Itanium. They probably also have one for PowerPC for running XNA games on xbox 360. Porting the BCL to a target platform, and building a static linker is probably more of a task though.
It's also worth remembering the rumors that Microsoft will offer a free version of Windows and an iPad Office. It seems like Nadella is looking to bring a new focus to Microsoft besides "Windows everything," so they may indeed have a newfound interest in cross-platform support.
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.