From what I believe, or at least think, didn't Carrey said that he'll retire from acting with the exception of Sonic movies? That Sega/Paramount will let him go when he asks to and won't recast Robotnik over it?
He's retired but not making exception for Sonic movies, he just said he might reconsider it. It could be interpreted he's just being nice in words while in the reality it's still a no.
And since so far no words of his willing to come back, the producer just went along with another villain instead of Robotnik since they don't want recast him.