I think the reason why the BCCI does not allow their players to play outside leagues is that first of all, they don't want to raise any potential competition and secondly, they are paying a lot of money to a lot of people in power and in that way they are doing what they think is best for them. Obviously, if they let the players play in other leagues, those leagues will also rise in popularity and maybe give IPL a good competition but by doing this they are eliminating any chance of that happening.
So where are the regulators in this case i.e. ICC? ICC stop cricket for IPL but cant stop such monopolies of BCCI. I also read here that BCCI never allow his men to take part in any other league but allow his female players to play WBBL and the hundred. If such thing continue to happen then rest of the world will never take this sports seriously.
If we say that cricket should move to league structure like that of football then we must have strong regulator who can overlook matters like FIFA is doing in football.