Why not run Zune on the Windows Mobile OS?

I read a rumor over at gizmodo.com that Microsoft may be releasing a VOIP/WiMAX/4G Zune player phone thingy. If I were a betting man (well, actually I am) I would say it will NOT be running Windows Mobile just as the current Zune does not. That leaves the question, why not? To me it seems much more expensive to create and manage so many different operating systems so why not just standardize on one? Apple is telling us that the iPhone is running a version of OS X. Ok, so I know XP/Vista will never fit on a phone (especially with the little amount of RAM included on the phones) but why not have all portable devices run Windows Mobile?

As I mentioned in a previous post, I would love to drop my iPod and use my Windows Mobile device to play music. The problem is WMP's IU is awkward. If they ran the Zune UI on Windows Mobile they could release the app as a "Zune Lite" for Windows Mobile phones and I would be a happy camper. Or someone could create a cool music player app, hint.. hint.. (BTW I have not found one with a great UI, if you find one point me in the right direction).