I don't think it was so much a problem of the implementation language, but rather that every feature needed to be present in both the compiler and VS (for synta

I don't think it was so much a problem of the implementation language, but rather that every feature needed to be present in both the compiler and VS (for syntax highlighting and IntelliSense) and those have been completely different implementations (I think highlighting and IntelliSense even were different parts). So you had to touch two or three completely different codebases whereas now you only have one.