Because ATT are greedy bastards who will only give you the reduced price if you sign up for Next? Which is a pain in the ass to bother with. I don't want to have to fill out a bunch of forms to buy a financed phone blah blah. I want to hand you a damn credit card, walk out and that's that. Wash rinse and repeat next year if I wish.
I can understand your trepidation, but AT&T have simplified their operations.
For example, I upgraded three lines on launch day at an AT&T store. I traded in a couple of old 4s phones too. No forms to fill out.
I paid off all three i6s a few weeks later. Did it at home via the AT&T app. Took a couple of clicks and was done. No phone calls to CS. No forms. No bother at all.
I'm in the process of unlocking them all right now. All via the AT&T website. Easy.