Ximian and Mono were built almost immediately after .NET was released and provided a decent multiplatform community-supported alternative for years, but you somehow think that forking an existing and working codebase would be more difficult than a completely independent reimplementation?

Look, there are obvious differences in how Java and the JVM are maintained and evolved as compared to .NET, but to continue claiming that these differences really matter all that much is just FUD, to put it politely.