I'd guess it was written in C for the sake of making it more maintainable.
Assembler sucks badly for good code maintainance. Bug detection and stomping is much easier in C.
-edit-
Anyway, the time critical parts probably are still assembler, since stuff like the task switching mechanism needs to do low level stuff that C itself doesn't really cater for.
As for speed differences, I bet you'd be hard pressed to notice anyway.
