Apple signed an exclusivity agreement with AT&T. Chances of it showing up on T-Mobile anytime soon are probably not zero, but probably close to it.
also the way people use them on carriers other than at&t, is by unlocking them, which voids your warranty and pisses off at&t. germany may have a law against selling locked phones, so apple offers a factory unlock, in which the phone is always unlocked, even after restore.