Because the LCCs started with pre-NextGen 737s so the NextGen 737s are the logical upgrade path for them. While the 757 is a more efficient plane if flown fully loaded, it would result in a mixed-fleet and tierd pilots and all the other...baggage...that keeps the Big Six's costs so much higher.

Asian airlines seem to want dual-aisle planes (Boeing and Airbus), so the 757 never took off. The only airlines that the 757 seems to make sense for is the Big Six, and while they bought a thousand of them, the plane is just no longer viable since they either can't fill them, or fill them at RASMs below CASMs.

[Edited for spelling]

[This message has been edited by SEA_Tigger (edited 07-16-2003).]