Arg, my searching is not working well so I'll 'splain for ya.
Firstly, two arenas are needed now because of the growth of the game, that is a given. Secondly, players are magnetized to the arena with the most players, which is a constant domino effect problem so offering two arenas with free choice is not enough. It will always end up as 600 players and 0 players. So HTC had to be creative in coming up with a way to force players to balance the arenas since we cannot do it on our own. You can still get in the arena of your choice if you try hard enough but around noon every day, it is difficult.