Care to name some of these platforms which Java doesn't run on which .NET supports? I'm having a hard time coming up with even a very short list.
.NET Microframework makes it runnable on IoT.
I'm also pretty sure that embedded Java is widely deployed in IoT applications.
PS4 (MonoGame/Mono) XBox One (.NET Core on the future and MonoGame/Mono)
At least you can run c# code on those platforms. Java though is unlikely to ever happen.