Couple of reasons. DALPA attempted to capture the efficiency gains from the 727 which it more or less directly replaced* and Southwest airlines actually made a convenient benchmark (and who said we could not pattern bargain off the Southwest student Council?).

We again tried to capture the efficiency gains over the 757 that the 737-900 is replacing. That was a dollar.*

*(however, considering the 737-900 needs a flaps 15 War Emergency Power takeoff to get out of a sea level airport with a 10,000 foot runway and cruises at 25,000 feet, we have discovered the 737 is less efficient than the 40 year old design it is replacing so in C2015 we are going to take a $15 an hour concession to the 757 ... but the trips are better ... the 737-900 can only go to former SAC bases ... which is why the zoomies all bid the thing ... reminds them of their youth )