Back prior to Mist of Pandaria, 71+ used to be able to 'glitch' their way into arenas if you will. Method was to stay in an arena team before leveling past 70, having someone who is staying at 70, ( preferably an alt ), to keep leader ( Since after leveling past 70 you wouldn't be able to invite or pass lead. ) Now you just get kicked out of teams after leveling
Henceforth, the only reason to level beyond to 72 or any other would merely be for items such as the nifflevar axe, or any learnable abilities worth your while; with the sacrifice of not being able to arena.
Is this for US or EU, the bracket isn't "dead" it's still very active, there's many 70's currently playing US and i hear the same thing is going on for EU, but mainly arena's is played at 70.