Its sad, but the largest airliners such as the 773/ER, A346, A380, the potential 747A, and the longest range airliners such as the 772ER and A345 are of little interest to US carriers
In most cases the US airlines simply do not need the seats offered by the larger variants due to multiple frequencies on dense routes which support hub operations (4 763s per day between ATL
works better than 2 747s for example) and due to market fragmentation (instead of flying 2 A380s per day between SFO
and Tokyo, lets instead offer 1 A330 per day to Tokyo from LAX
, etc).....the market fragmentation theory was the basis for Boeing's decision not to launch a direct competitor to the A380. These two concepts, taken together, has reduced the US carriers' demand for large airliners....the only exception, of course, are certain trans-pacific routes and even those markets may fragment over time. This situation is rather unique to the US at the moment, where there are multiple carriers operating long haul flights from multiple hubs...thus, US carriers seem very happy with their 763ERs and 772ERs and need nothing larger.
The ultra long haul variants have not caught on mainly due to price (these are very expensive toys), risk issues, and lack of suitable routes....ultra long haul is not a priority for the US airlines at the moment.
Times change, when the 747 program was launched, every US carrier rushed to order the type....then again, those same 747s nearly put a few airlines out of business.