My guess is the difference goes back to the days of the begining of pax jet operation.

If I recall correctly:
Convairs hauled up to 70 pax
B-337 Stratocruiser 50-90 pax
DC-7 100 pax
707 hauled up to 180 pax
DC-8 up to 250 pax

So when jets came on the scene, payloads doubled. Also, being a new technology with higher performance, pilots could demand much higher pay.

Just a guess.....