It's much harder to get enough of a .NET runtime together for a new platform than it is to lean on a platform compiler and binary toolchain which must already exist. It's all very well saying "just download the installer," but that won't get my nemerle code running on my phone.
Nokia N900. Ok, so there's Mono for Maemo, so that's not a particularly convincing argument in itself. However, the point stands - replace Nemerle with Scala, for instance, and I'm stuffed (mostly).