I started to use Mr. Roger’s Neighborhood as an example, but switched to Sesame Street for effect since the former “only” has 922 episodes. The point is, PBS does the same thing with all their shows. It doesn’t matter who makes it; everything from Arthur to Word Girl only has a dozen or so episodes available at any given time.
(Ditto for non-kids PBS stuff like Nova or This Old House, for that matter, I think. Even on Youtube they only make the latest few seasons available and remove access to everything older in hopes you’ll buy DVDs or something.)