AT&T vs. Apple
Plain and simple AT&T hates Apple.
AT&T does not like iOS devices or Users. They want to make the iOS experience the worst it can possibly be, so iOS users will migrate off the network.
There is no other explanation.
This is not the first time AT&T has been at war with a vendor, have you notice how few Motorola phones are on AT&T, after the Razr exclusivity they fell out too. Seriously, most stores don't even have active iPhone and iPad on display, just some cardboard cut out.
The problem is the iPhone/iOS money pot is so big they can't truly say goodbye. This is just a bad romance, and the users are the children caught up in the (separation) divorce.
If Adele was an american she would be singing about AT&T instead of that guy. Apple
isn't helping either - constantly changing the SIM design with each major release.