ATT has no high end phone? Umm, iPhone? You and I may not think it is the cat's meow, and may not want ATT to bleed us dry, but that doesn't change reality that it is the go to device. ATT can sell 5 or10x as many iphones even if EVO and EPIC were flowing -- and make more for the next two years on each customer.
Scale and status matter. The scale of the iphone means one is learning an UI that is ubiquitous, that apps will flow, etc. Hey I think TP2 running WM6.x is better than iphone, but I also know why huge numbers of people that move the smartphone market place and market share prefer iPhone. It is part of an entire ecosystem:. itunes/ipod/iphone/ipad
Status also gives value to the buyer. It used to be that in business and social meetings that your watch, you class ring, in the case of women their jewelry etc were personal adornment sof status symbols. You may think that is unimportant, and I wish it were os, but Status symbols are important and do give a benefit to those with them. people find those adorned with status symbols more attractive (and I don't mean just sexually, I mean jattractive in all senses including business), more believable, etc. The iPhone still dominates and will convey top status. (I am not saying I like it but simply I know it). Even the fact that ATT costs more conveys status in some ways.
Also Sprint is not ATT or Verizon. Sprint is an also ran. It is troubled. The way its last quarterly numbers were spun as a win doesn't change the fact that they lost in contract customers.
Verizon will be serving 4g LTE to as many customers as Sprint's betamax wimax in a matter of months. Sprint will be in a very bad place when makers of handsets tell them: "no wimax version." Wimax won't be just a smaller but high demographic and robust alternative as CDMA is to GSM, it will be betamax. Some parts of conversion to LTE for sprint will be smooth and others are potentially VERY messy and costly.
In short Sprint's advantages in windows and opportunity are smaller and of shorter duration, transients, and it Sprint that needs its top end phones out there much MUCH more than ATT and Verizon.