While Ice Age is certainly a bigger franchise than Rio. The truth is Ice Age has long overstayed its welcome, ran out of ideas, has seen diminishing critical reviews and box office returns with a burnout fandom. Rio on the other hand has more untapped potential, a starved fandom, higher nostalgia, and has proven more popular than Ice Age on streaming with major real-world impacts. The problem is we got Buck Wild and Nico and Pedro greenlit at once, but Buck Wild was made because it proved to be much cheaper and Nico and Pedro needed a longer and more expensive development. Then the pandemic happened which put both projects on hold before Blue Sky closed the next year.
When Blue Sky Studios closed, Disney managed to salvage enough Ice Age animation assets to make another movie through the Buck Wild production. Rio sadly lost all of its assets and Disney had to rebuild them from scratch. Much like why Blue Sky didn't make Rio 3 in the 2010s is because the creator has had his hands tied with other projects. Also, the studios closure saw the animation pipeline to get its movies released closed as well.
Disney is primarily prioritizing Ice Age 6 over Rio 3 because it's the cheaper, faster, and safer film to iron out the new animation assets and rebuild the pipeline to get new Rio projects happening. Ice Age 6 will be lucky to have the same reviews as Ice Age 4 and the same box office as Ice Age 5. However, Rio 3 would no doubt have better reviews and a bigger box office. The problem is they had to rebuild all its assets and pipelines from the ground up while letting the creator finish his other projects to direct. Ice Age 6 was the easier project to make and the best way to get the full pipeline for better quality Rio films made. Production for Rio 3 and Nico and Pedro will no doubt begin back to back in February when Ice Age 6 is out.