LH used to offer a nonstop to Frankfurt, but that ended years ago. Seems like it might be worth looking at again now that PHX is a Star Alliance hub now (it was still America West back then).

Up to now, US didn't have an aircraft with the range. The A332 does, but US seems to believe they're better utilized from the east coast. One issue is that a single aircraft can't make the roundtrip in a single day, so either the flight can't operate daily or more than one aircraft is required.