[kevingadd]

To second what scott_s said, you'd have a huge interop layer. The fact that C# has a closer match to C++ types and primitives doesn't mean you have no interop. Unity has a huge surface area between the C++ parts of the engine and the C# parts of the engine; a ton of work goes into maintaining that, and it complicates porting to new target platforms like the web (can't just feed everything through emscripten).

[azakai]

Yes, C# and other .NET languages are nice, but the open source implementation (Mono) ends up being difficult to port to novel platforms due to a combination of technical and legal issues.