The only US majors that use the 747 (excluding freight operations) are those carriers with large and long-standing Pacific operations (United and Northwest). The basic reason for this is that, for the US carriers in the US domestic, Latin American, and Atlantic markets, the consumers prefer frequency and convenience over larger aircraft. As a result, a carrier flying from Philadelphia to Los Angeles, would prefer to offer 5 flight times using A319s and A321s over offering 2 flights a day using a much larger aircraft.
Advances in aircraft and range have made this possible now that smaller jets have the range that only their big brothers had just 20 years ago. This also is why many Midwest hubs (Memphis, St. Louis, Kansas City, etc) aren't as important as they used to be. They just aren't needed due to advances in aircraft range.
In the same manner as domestic flights, the Atlantic traffic is dominated by 767s and 330s because that allows for more frequency and it allows for non-stops from smaller cities. You don't have to force all trans-Atlantic flights through Boston and New York due to the smaller twins having the range to bypass these historic hubs. Logan and JFK
aren't as important as international jumping points and, as a result, are not as congested. The smaller jets actually can help reduce congestion since people are not forced to fly to airports they don't want to fly to in order to connect to flights. Also, the twins are cheaper to operate and maintain than three engine (MD-11s) and four engine (747 and 340) aircraft.
Continuing the trend, 777s and, eventually, 787s and 350s will replace most 747s on the ultra-long flights. People prefer the flexibility of additional flights and the airlines prefer to not put too much metal in the sky. Nobody lost money putting too little capacity in the air. Money is lost when you put too much capacity in the air.
Sorry for the long post.