Most existing C compilers (eg clang or gcc) are pretty large and complex beasts. Its often easier to start from scratch than it is to extend an existing codebase that you have never worked in before. It depends on your goals: if you want to appeal to an existing user base and use an existing ecosystem, then modifying existing compilers (or compiling to an existing language) is probably a good idea, but if that isn't a goal for whatever reason, then doing so may not be worth it.

Not saying this is a good thing, just that it happens (at least, in my experience).