Eh, my own reasoning for the first time and possibly-soon the second time isn't nearly as exciting as others in this thread. I just get tired of using the same couple of teams for months before I can get another team ready. Coupled with fighting the same few teams over and over. Makes the game feel really small and I can only tap on the same few menus so many times before just moving on.
Sorry that I can't pinpoint a specific timeline. It's just a general mobile gaming complaint, really. I'm hoping I can farm one more team that I have in mind.