Frank already had a lot of full time characters, 2 of which interacted with Kermit on a regular basis. That would be like Steve doing Rizzo and Gonzo, or Eric doing Piggy and Kermit. It just wouldn't work. Steve had like no characters and Dave really only had Gonzo. Frank had Piggy, Fozzie, Animal, Sam, and others.
Question, why is it that Frank has 3 main characters on the Muppet Show and on Sesame Street? Piggy, Animal, Fozzie / Grover, Bert, Cookie. That's kind of weird. Even Jim Henson didn't have that many main characters.