I thought cartoon network had a 7 season cap for how long a show could go on for
That would be very arbitrary. I can't see any network doing that - why would they not renew a show for an eighth season if they're going to make money off it?
I read something a while back about how TV contracts work, apparently the terms are very favorable to the networks in the short term, usually the first 4-5 years of the show, with the carrot being that the studios are promised a much bigger share of the profits if shows go longer. After that, the studios start to make real money. The networks really control the whole deal so they push these contracts that favor themselves in the short term, since they have the power to cancel the show at any time. Often it makes more financial sense (for the network, not the viewers or studios) to cancel a show near the end of this initial contract period even if it's still getting good ratings, because they can renegotiate a new contract with the same or another studio to pick up a new show for cheap.
So, it's a gamble between the networks and studios, you want your show to be so popular that they can't cancel it. "Just a little" popular doesn't cut it because they've got other studios desperate for the work with slightly shittier new shows they can replace your one with and cut costs.