How would you rotate/shuffle shards? Arena shards are made up of players that entered arena at about the same time. That's a pretty easy and fair system. If you start shuffling shards randomly you give some players big advantage over others based on when they started playing..
You cant really start making arena shards based on player's gp or roster either - you would end up putting all top players/spenders against each other... that would negate any reason to spend on the game or the sense of pride and accomplishment from getting to the top..
