Apple relies too much on its hardware for income. If they release OSX on anything other than more expensive, Apple hardware, people would use the cheaper, non-Apple hardware and Apple wouldn't get the money from the hardware.
I've also heard arguments of whether all that new snazzy interface stuff could even effectively run on an x86, though I'm not sure how you could substantiate such a claim.
Ages ago, Apple tried PPC clones (or was it before Apple made the jump to PPC?) and they got hurt badly for it.