Lufthansa used to fly 747s to DFW as well.

The runway's at DFW are plenty long enough to handle them. Why so few? Take a look at how few foreign carriers serve DFW.. Dallas is in the middle of the country. It's not on the Pacific coast like L.A or San Fran, so it doesn't attract the pacific rim folks. They just fly to LAX or SFO, then fly American or whatever to Dallas. Same with east coast and Europe and western Asia.