Nobody knows for sure. The most visible reason is to begin a transition from a desktop-only OS to one that can bridge multiple device types and thus stay relevant and plugged into where the biggest growth is happening. The other reason is to start a move away from the archaic Win32, which is very much tied to x86, and be able to move a little more swiftly with different architectures, namely ARM chips.
Well, looks like you want something lower level then that offer you much more control. (Or, you want to purchase Unity and source) I would consider looking at Axiom. It's a port of OGRE to C#, rides over Mono. Mono itself has C# targets for Android and iOS. One thing I've not found, but not looked for, is if someone has combined all these things into a really portable engine that usues C#. I happen to like that language, generally-speaking.