It comes down to device drivers for specific hardware in the device. Each PPC is customized by the manufacturer with device drivers for the specific hardware they choose to use in the device (while general specs are all the same, there are differences in interfaces, flash storage, HD, chipsets and wireless radios). No one has written the OS customizations (device drivers and etc) necessary to run Windows Mobile on the LifeDrive.
And since one wouldn't have a license to install WM on a Palm, it's technically software piracy. Unlike Windows XP, you can't go out and buy a copy of Windows Mobile and the associated license key. That's one of the reasons why we've seen occasional Linux ports for PDAs in the past (generally for an older model of iPAQ) but not other platform ports.
A shame the rumors of a Dell Axim with hard drive turned out to be just rumors. Perhaps someone will make one someday.