AT&T doesn't believe in the Nokia Lumia
Nokia blew it by signing the Lumia phones over exclusively to a carrier that doesn't care. They just flushed their chances in the US market down the toilet. AT&T will only sell the Lumia 820 in black and the 920 in black, white or red. Very little to make the phone stand out on store shelves. They also likely won't carry most or any of the Nokia or JBL accessories for Lumia. The fact that AT&T doesn't believe that the Lumia phones have enough appeal to carry the colorized versions of the phone in their store means they don't think it will sell that well.
AT&T is an iPhone carrier, they sell iPhones and iPhone customers are happy with their phones. The only chance Nokia had in America was through a broad strategy that put their phones on as many carriers as possible, similar to the Galaxy S. This was the only way that Samsung was able to compete with Apple in the US market and it forced Apple to end their exclusivity deal with AT&T. Now Nokia signs their company over to AT&T just so that AT&T can convince investors they're not just an iPhone company.