I'll put a foot in my mouth and venture a guess that in the later years more long cabs were available for all trains. Crews probably liked them as they have a bunch more room.
Seems like the small hacks were used on the branches.
Also, while the locos were pooled, I recall that cabooses were assigned to conductors. If conductors had seniority like engine crews, than that reinforces my first statement, as the guys at the top of the list likely rated long cabeese, and were thus more apt to get called for a train.
K