Only 3 years? 28 episodes?

The same thing was done with Land of the Lost. I noticed that Sid and Marty Krofft shows as well as Filmation shows all did the same thing; film only one or two seasons maybe three, then rerun them for the rest of the decade. In answer to your question, I suspect it had something to do with budgeting issues as all of those shows operated on very little money. I don't think ratings were the reason as most children in the 70's tuned in week to week in large numbers and adored these shows and characters.

Saturday morning shows were usually conceived for 13-16 episodes (depending on network and timeframe). You can't apply prime time season length to Saturday morning programming. Shows were very rarely continued, even when successful. They would just re-run the show. Sometimes, they retoooled things into a new version, as happened with the various Scooby Doo shows. The fact that this had 3 sets of episode orders shows how successful it was, not the reverse. The number of shows that got extended were just a handful. Off the top of my head: Shazam, Scooby Doo (the original show), Tarzan, Lord of the Jungle, Superman (the original Filmation show)and Flash Gordon (though that might as well have been a new show, for the changes). So, Filmation did it more than Hanna-Barbera.

As for the number of subsequent episodes, they would factor in rerunning the previous set, along with new episodes, so the orders were smaller. You have to remember that the Saturday morning audience turned over each season. Kids outgrew cartoons and moved on, or outgrew certain types of cartoons/shows. So, the networks were constantly looking for new material. Budgets were another reason. You didn't have the same kind of advertising money, so budgets were smaller, especially live action shows.

"Fortunately, Ah keep mah feathers numbered for just such an emergency!"