That makes sense. It very similar as to why we saw less Rowlf on TMS. Jim was too busy with Kermit and it was hard to include the characters together.
For some reason, Steve seemed to be closer with Rizzo than Kermit for the longest time because, well... Rizzo is his character. if you have the MFS DVD, watch the commentary track, and Kermit leaves the theater before the opening number and gets replaced with Rizzo up until the end credits where Kermit randomly comes back. That's all the proof I need. Steve really seemed to be busy with Kermit primarily, possibly with Link and Beaker (even though the two share a sequence in the Barber Shop Quartet, but that's recorded anyway).
But that doesn't explain Fozzie/Piggy's ability to always be in scenes together throughout the whole of Muppet history.
Actually, MFS answers the question again. While there is dialogue looping, in scenes with all Frank Oz characters, they barely have any lines and the chemistry seems a little off. Even if someone else played Rizzo, the dialogue would have been looped and he wouldn't have all that much to say, or to interact with. There's similar problems in LTS, as Rizzo comes with Kermit, Pepe, Gonzo, and Fozzie to the North Pole. Rizzo seems to talk a lot less, then Kermit seems to have less dialogue, then Rizzo barely says anything again.
Still... I wouldn't worry much about 1 project. Other than Rizzo's very short Munchkin performance in MWOZ, he was prominently featured in all the other projects.
Maybe Steve is just so relaxed as Kermit now, he can use him as his primary character now.