Kent is just a tough place to win. They schedule 3 games a year against the likes of Alabama, OSU, Clemson, Oklahoma, etc. That means they have to go go 6-3 in all other games just to make a bowl. Their facilities are absolutely terrible. As I understand it what they budget for assistants is near the bottom half of the MAC. Their AD publicly has come out and said the athletic programs focus is basketball.
Lewis likely just felt he achieved all he could at Kent and it wasn't even enough to get him into the discussion for what I'm guessing is his dream job at his alma mater (Wisconsin). He probably feels going back to being a P5 OC increases his opportunity to get where he wants to be.