Tigger,
it's actually more simple than even that, so far as the Big6 go.

The 757 was first introduced as the transcon ship par excellence... and they majors used it for just that. Upon the arrival of the 737NG, the majors now had at their disposal aircraft which could operate the same transcon and short hop distances as the 757, but was easier to fill and could therefor translate into higher frequency (which we know is worshipped as one of the highest gods in aviation mythology)