I don't think that Apple is positioned as a software company. Their profit comes from the hardware. To be really successful in the desktop market, historically, you need to focus on one or the other.
If they had OSX for the PC, you'd incur huge driver development costs, some new code, more security exploits, a shinny new target for spyware, a bigger support network and most certainly, unhappy customers.
Plus, there are several (Linux, BSD, Solaris X86) *nix solutions for the PC with which Apple would have to compete, not to mention Windows.
