What do you do when you’ve exhausted the movie franchise to Planet of the Apes? In the mid-1970s, reboots were unheard of. They did remakes, but all the Apes movies were too recent. And anyway, Battle for the Planet of the Apes did not do well in theaters.
Instead, they made a TV series that was gone by the time Rudolph the Red-Nosed Reindeer started airing in 1974.