The gRPC libraries are primarily developed by Google. Google pours most of its development resources on its core languages: Java, Javascript, Go, C++, and Python. Google then allocates a developer to port the implementation to other languages and platforms. C++, unlike Java, is not typically used for cloud development, so I don't think a lot of resources were allocated for the language.
You can see how the difference in implementation effort can impact performance by comparing dotnet_grpc (Microsoft's fork) against csharp_grpc (Google's original implementation). There's more than a sevenfold improvement in req/s in Microsoft's implementation in the 1 CPU server case (35070 vs 5337), outperforming nearly all the Java benchmarks.
Also, many of those top performing JVM implementations are the same code running under a different garbage collector. .NET has two GCs each with a parallel option, but we only see one benchmark using likely the slower GC (Workstation GC instead of ServerGC).