Additionally, the amount of film that is analyzed prior to each game, as well as the amount of free-agency these days means players are so familiar with one another and opposing team's play books, that the technology already exists - in their brains.
Maybe it's the football purist in me, but I really don't think iPads would add much. In the NHL, the Maple Leafs have an iPad or two on the bench during the game (the NHL has no restrictions) and they stink.