The problem there is that it would be wide open to exploits, in a game which is already pretty heavily metagamed - what's to stop someone loading their team up with dead cap then abandon it, and retake it a tick later? Or abandon a team and create a new user (against the TOS, I know) who then takes over the team.
(That being said, I don't know if the open leagues require admin approval to take over a team.)
Adding a new exploitable avenue to those leagues which aren't being actively managed would end more poorly than the current situation.