A language is much more than just syntax.
With mono, one ends up writing C# code similar to C and C++, with #ifdefs, or having to search for .NET libraries that are cross platform.
UI for example. Pure Java UIs plain suck; you can use Gtk# in C# for the same thing, but none will be up to the platform's UI standards.
Besides, the JVM isn't available on iOS and Android, so it isn't universally available either.
A language is much more than just syntax.
With mono, one ends up writing C# code similar to C and C++, with #ifdefs, or having to search for .NET libraries that are cross platform.