The rationale is, that once you have a programming language that is implemented in itself (usually that means that the compiler/interpreter is written in that language), then it means that your language has tackled and can deal with many issues that "system languages" deal with. Mainly dealing with the OS in a lower level, dealing with CPU/RAM, etc.

This is a large problem space that you can glean over by using C as a layer of interaction between your language and the underlying machine, but it makes your language a: not truly a "system language" and b: it also ties you to C philosophy, API/ABI, calling conventions and so on.