Apple wants the Macintosh to be seen as the system of choice for music production They would be very much shooting themselves in the foot by releasing Logic for Windows, as currently it's one more reason to buy a Mac for making music. Additionally, there would probably be all manner of speculation about them "handicapping" the PC version.

iTunes is released for the PC because they want everyone to buy into it, Mac user or PC user. If they make it Mac only then PC users will just go and buy their tunes elsewhere. No one is going to go and buy a Mac just to get iTunes.