That's because LAX and JFK still haven't completely updated their international terminals to modern standards. This is totally unlike SFO (San Francisco International Airport), which has a totally new International Terminal that opened at the end of 2000 designed to support more 747-400 operations and even has four gates that are ready to support the A380-800 right now.
By the way, I've heard that Airbus will start taxiing tests on the A380 prototype within the next few days.
Another problem for SFO is that the runway length is marginal for the 380. You can thank green whackos and NIMBYs for that one.