If I put on my silo-loving cap and ask... If MS port the CLR to the Mac, or even Linux, does MS benefit more by being able to sell more copies of VS to Mac programmers, or the Mac platform benefit more by being able to run .Net apps.
On the other hand, "porting" has a non-zero cost. We all know that there are a lot of call-throughs in the CLR that rely heavily on the underlying OS. Look at the difficulties Mono is having (albeit a very small group of laborers).
Maybe we should ask why isn't Apple help out more with the Mono effort since they really have more to gain than MS.