There are a lot of how ideas that Jim Henson pitched to networks in the 1960s that didn't get made, most likely because the networks weren't interested in them. At the time, most networks thought of puppets as being a kids entertainment, and didn't think they'd hold an adult audience. Sure, the networks knew that the muppets were good enough to be an act on an adult variety show, but didn't think there'd be enough ideas for a weekly prime time series starring them.
Some of the shows that Jim Henson wanted to make did get made into pilots but nothing beyond that. One example is Tales of The Tinkerdee, which actually had two pilots produced (the other one being Land of Tinkerdee), but a series was never made.
It's possible that by time The Muppet Show was made that Jim Henson wasn't interested in The Zoocus anymore. Or maybe he was too busy with enough projects to try to get The Zoocus made. In the 1980s, he was busy with Fraggle Rock, Sesame Street, The Ghost of Faffner Hall, The Jim Henson Hour, and The Storyteller, plus various movies and specials. And the only ones of those series that he regularly performed on were Sesame Street (and that was only in segments so he could have spent only a few days a year on Sesame Street) and The Jim Henson Hour.
I think Jim Henson might have abandoned his idea for The Zoocus before Sesame Street began. Either that, or when he started pitching The Muppet Show to networks (it has been said that The Zoocus was similar to The Muppet Show).