Polyglot dev here, that uses both ecosystems, Java since 1996, .NET before it was announced to the public in 2001, only available to selected Microsoft partners.

A big difference between both ecosystems is that the Java world is like C and C++, even though Java isn't defined by ISO or ECMA, since Sun days the main implementation is only a reference, there are official documents for everything, and there is a plethora of implementations, with various kinds of JIT, GC and AOT approaches.

You can pick the real time versions for embedded from PTC and Aicas, the cloud first from IBM and Azul with finance markets in mind, the Android cousin, the various implementations for M2M gateways, copiers and phone dashboards (Ricoh, Xerox, Cisco), IoT with microEJ, and many more.

Whereas Microsoft hardly cares about ECMA nowadays, most of Mono/Xamarin is gone replaced by Core CLR and modern .NET, .NET Compact is gone, community maintained and so on.

That alone, regardless of the languages on top of JVM, or CLR, makes a big difference on the audiences when one silos themselves to a single ecosystem.