Also as a single-person creation, Linux is basically the implementation of an API which existed before -- not only did this bypass a huge amount of work that would go into designing a fundamentally new operating system, but it meant Linus's work would be useful because lots of existing software would run on it.

(I find it depressing that real innovation in operating systems seems impossible because everybody wants to run old software... Thus all the "bloat" has to get put back into the system.)