Because the magic in the iphone and ipod come primarily from software (and for the ipod, the music licensing relationships that apple alone had in the beginning.) Android hardware can be pretty good; the app ecosystem, the ecosystem in general, the clear love that Apple has for the iphone, and the control that is (generally) used in user-beneficial ways all make the iphone superior to android.
in the case of the iPod I'm not sure that was true. remember, these were the days of everyone having a bunch of mp3s on their hard drive. The actual iPod software was extremely rudimentary, and basically barely integrated with iTunes. this was still in the days of windows, keep in mind, and before apple actually launched iTunes for windows. and still, it was immediately a dominant player and not one company was able to launch a competitor _before_ apple built iTunes for windows.
The iPod wasn't that popular before Apple launched iTunes for Windows, though. So I imagine competitors were considerably less interested in it as a target.
Also, one of the iPod's major interface innovations was the click wheel - IIRC they've patented the hell out of that, so no one was about to launch a competitor.
