Because I want to be self hosted
But you might possibly have found that one weird corner case where it doesn't make sense to depend on a mighty, highly-sophisticated, industrial-strength, extremely well tested and supported, nicely mature, compiler tooling framework.
But that's a pretty big exception to most language implementers who get all of that and more for super-cheap compared to implementing it themselves.
If you're writing a C++ compiler, self-hosting LLVM might not be a massive additional complexity. For any other language it's not an option if self hosting is one of your goals.