OK, here are my theories as to why UPA happened, and why animators abandoned what they were good at. They might not be right, but at least it's a stab at an explanation for something so crazy to have happened. Maybe you have some theories too.
1) SOME SKILLED ANIMATORS TAKE SKILLED ANIMATION FOR GRANTED
By 1948 there were many many brilliant animators who could do totally sophisticated rich cartoon movement. Their animation and drawings had level upon level of intelligent artistic principles, ideas and planning in every scene. To the audience it would be magic. Maybe by the 40s this brilliant stuff was so easy for them that they started to take it all for granted. It was second nature to them, so they were no longer impressed by their own sophistication.
I know that in the 80s, I brought some tapes of Clampett cartoons to Bill Melendez to show him.
He had previously told me how proud he was of his UPA work and how revolutionary it was. I seem to remember that when he saw the Clampett stuff, he had forgotten how amazing and sophisticated the animation was. In 1985 (as now), any animation from the 40s would look like superhumans from space did it, because everything had become so primitive by then.
We told Melendez how much we loved the 40s stuff and he then agreed that it was the best animation ever done. The UPA stuff was more about the design and just doing something different and he was still very proud of that. He said it cost as much to do a limited animation UPA cartoon as it did to do a fully animated funny cartoon. I still can't figure that out, but of course I believe him.
Maybe some of the animators who could do such amazing feats of cartoon magic just got bored with their own skills and wanted to do something different.
2) SOME ANIMATORS CRAVE SERIOUS RESPECT
Real cartoon animation-the full stuff of the 30s and 40s is very sophisticated, but only animators would know that. The audience and the critics just thought of it as cheap throwaway entertainment, and as history teaches over and over again, entertainment and fun doesn't get serious critical attention no matter how obviously skilled it is. It sure doesn't win awards.
Some animators and directors needed to know they were doing something that was above cheap mass entertainment, something that had a higher meaning. Something that could get respect. Some cartoonists wanted to be "artists", not realizing that they had previously invented a whole new artform that could do what no other was capable of.
Pleasing hundreds of millions of people around the world with obvious astounding skill and talent is not good enough to get you respect.
NEXT...2 DIFFERENT WAYS FOR ANIMATORS TO BEG RESPECT - DISNEY'S AND UPA'S