Apparently that was the standard for a Saturday Morning show back in those days. I mean, if you look at some of Hanna-Barbera's shows from that time, I think many of them had that same number of episodes per season as well.
Then again, back then, primetime shows seem to fluxuate their number of episodes per season from like 26, 30, 32, 36, 38; I don't think 22/24 became standard till the early 70s.