The team needs a break. Hell, even a new Super Monkey Ball would be a change of pace.
The teams alternate between two.
But that's the nature of the industry now, I don't expect new IPs from them after Binary Domain was a major financial failure. I'm also not sure how rigid their team structure is considering people from Sonic Team helped develop the Black Panther spinoffs.
(And I for one, welcome the endless amount of sequels. I need my daily dose of food cart submissions.)