Ask HN: Intel Kernel Vulnerability fix: why software not microcode

Could someone please explain why the mitigations for the Intel CPU bug are having to be implemented in the OS rather than Microcode?

As it only seems to effect Intel and not AMD processors, and as the issue seems to be with the CPU not invalidating cache after a failed OP, this seems like something that would be best fixed actually on the CPU itself.

I assume there's good reasons why not; if anyone could explain it would be greatly appreciated.