Why some kernel actions cannot be written in C

I apologize if this should sound trivial and unsubtle but I couldn't figure out an intuitive way to google it, Why are some kernel actions like saving the current state of the registers and the stack(just to mention a few) written in Assembly? Why can't they be written in C because after all, presumably, when compilation is done, all we get is object code? Besides when you use ollydbg, you notice that before a function call(in C), the current state of the register is pushed to the stack