I was among the first to criticize the Lumia 920 being an exclusive in the US, but as time has gone on, I'm not sure they had another play available.
People yelling "Well if HTC could get their 8X on all carriers, there's no excuse for Nokia!!!" clearly don't see the irony that HTC DID NOT get the 8X on all carriers. They didn't get it on Sprint, just like NO Windows Phone manufacturer has gotten a WP8 device on Sprint.
The manufacturer that everyone uses as a model - Samsung and their Galaxy S III and Galaxy Note II being on all carriers - have a Windows Phone flagship that NO US carrier has laid claim to as of yet.
So, clearly, it's not just a matter of will. Nokia worked WITH each willing carrier to make a 820 variant. Verizon has the 822. T-Mobile has the 810. AT&T has the 820. If they couldn't do the same with the 920, it was probably due to demands from AT&T. They obviously aren't averse to trying to get their devices on US carriers (read: AT&T). But they can't force them to carry it.