I don't think anything "went wrong" with Gonzo and Camilla's relationship, they've just been doing a lot of adaptations lately and maybe there just hasn't really been much of a role for Camilla right now. Their relationship was featured in the Muppets Wizard of Oz though! So I believe they are still going strong.
As for Rizzo and Gonzo. I think it could have just happened that when Gonzo was assigned the narrator in MCC, they needed to give him a sidekick for comedic purposes. Gonzo often needs someone to play off of to really enhance his personality. And Rizzo seemed like the perfect fit. Their chemistry worked so well they continued it to MTI as well. I also believe it has a lot to do with how well Dave Goelz and Steve Whitmire work off each other. They've had great chemistry since early on with Fraggle Rock also.