mmmkk,
It is a good idea and a fair question, also one that has been brought up with in the CM Team before.
DD has pointed out most of the reasoning behind remaining a 2hr event.
In the end, it is just to much to ask of the Set-up CM's and even most of the players to become a 3hr event.
Also in most cases the 3rd hour would see a dramatic decrease in players in the arena.
Remaining a 2hr event also keeps with an old saying in show business.
"Always leave the audience wanting more."