They need serious slot shortages and very large passenger numbers, one reason why LHR has so much A380 service. BTW, LH and AF will begin JFK service soon and EK plans to resume JFK service also; those according to various reports. Lots of A380 capacity is being diverted this year for Johannesburg to serve the World Cup.

Most US carriers have had a tough time making money with very large aircraft. PanAm, TWA and Northwest were the champion 747 operators in the US after all.

The long thick routes seem to be making a comeback though, possibly helped by the strong positive reaction to the A380 so far. It would be a big surprise to see US carriers opting for very large aircraft any time soon. A surprise I'd like, but a surprise. It would also be more efficient that running high frequencies on routes such as JFK-LHR, LAX-JFK