Both UA and NW have significant Pacific longhaul routes. For a long time, that's the only plane that can fly those long distance, and economically. You'll find that UA/NW don't use them on any other routes they have, including those to Europe.

But the 777s can do most of those routes more efficiently than the 744, except for US-Australia. That's why more and more UA Asian flights are on 777, and NW're using the 332 for the shorter Pacific routes. In fact, NW discontinued their JFK-NRT flight last year, after running for decades and decades. They can't compete with UA/AA/CO's 777 in efficiency, and NW doesn't have other equipment that can do that route.

Neither UA nor NW have the cash to replace too many of their planes. And I beleive both have 744s sitting in the desert.

No US passenger carriers are interested in the 380 right now. FedEx has ordered it.