C# (largely inspired by Java) run on even more platforms, because there is compiler/runtimes for mobile platform. It’s also the second biggest "entreprise" language, which fix a lot of Java pain points.
In practice, how many developers write C# on a non Windows platform? I'd say a very, very tiny minority.
On the other hand, Java is being written on all platforms and being deployed on many as well.
Plenty do - think of deploying c# web services on Linux servers / containers.
I write C# on Linux and I am basically the only person I know who does that.
Whether that effort pays off, only time will tell.
You can also compile JavaFX apps AOT for iOS! It's called Gluon Substrate, check it out.
Java really does run on a lot of stuff, even if OpenJDK itself may not.