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).