Well, as I remember it from when I wuzza kid: 20 exposures for home users to fit budgets, as mentioned. No doubt it had something do do with its development in countries that used metric system. 36 exposure rolls were thought of mostly for pros back then, although one could save substantially by using 36 exposure rolls.
I do remember that photofinishing at a later date was very accustomed to dealing with 12 exposure rolls from other sizes, e.g., 620, 127, and 120. I seem to remember that it was an accomodation to the photo processors because it gave them a little boost in dollar traffic/profit and kept them in 12s. 36 exposure rolls fits the 12s rule, too, but most people still did not like them. Even later on, some then-newer sizes, like Instamatic 626, were brought out in 12s at first.