Maybe someone can enlighten me; but why does AT&T seem like a popular launch partner with new mobile handsets? The only reasons I can come up with are a) money

Maybe someone can enlighten me; but why does AT&T seem like a popular launch partner with new mobile handsets? The only reasons I can come up with are a) money (AT&T taking a smaller cut of subsidies/paying the manufacturer more for the device) and b) "portability" between the US provider and International providers. Are there any other reasons? Or is it really that simple?