Beacause I don't relish the thought of shelling out over $900 (unlocked iPhone 6, 128 GB, $850 + 8% tax) any sooner than I have to. Even if I got one subsidized, it would still be about $500. This isn't pocket change.
"there's nothing that would stop you from buying a Verizon, AT&T, or Sprint device at the Apple Store and coming over and putting a T-Mobile SIM in it,"
Umm, except for the fact that AT&T iPhones are SIM-locked for the duration of your contract.