Primarily because it’s fun. But beyond that, how else can you boot .NET on a floppy or small USB stick? Who else will try to put .NET on the Wii, OLPC, and iPhone?
We are also developing a TCP/IP stack. Imagine instead of deploying half a dozen virtualized OS’s, deploying many dozens of dedicated OS’s. One that only does DNS, a few that only do HTTP, etc. One instance, one function.