To make a good ranking system requires a lot of time and testing. I think it also cost money to improve the game in such a way. Also, PL is supposed to be a kids game, not a competitive game(some people may think otherwise). In my opinion, they shouldn't waste time on a ranking system, but rather spend time on fixing current issues. Maybe in the future if the game suddenly gets more players, they might get more profit from PL, then they should make a ranking system. Just maybe. But for now, they should not.