Because it's Apple and they have big huge public releases and built up hype that loads and loads of people queue up on the night of release and buy all of them straight away.....this never happens with anything else (except Harry Potter books, but same thing goes there
)
You'd never have every shop selling out of a new Nokia straight away.....because people just go and buy them normally at a normal rate, rather than rushing out to instantly get the latest offering from the almighty Apple
And no, I'm not an Apple hater, I've got an iPod touch, and MacBook Pro and owned 3 other iPods