It wasn't. You typically bootstrap a compiler by first writing a compiler in an existing language. Then you get a compiler that can sort of compile your langauge, but the bootstrapping language contaminates your assembly. So you iteratively compile the compiler with itself until it's purely in the new language.
The very first compilers were bootstrapped from handwritten assembly. C was famously bootstrapped from B. GCC started in assembly I think and has been compiling itself for decades.