Altitude, 911 flights had just taken off. Leave your cell phone on next time you fly and see when the signal drops off. But at 35,000 feet no cell phone signal will reach a ground tower unassisted.
Not true for the one which crashed in Pennsylvania.
Actually, the problem isn't lack of assistance. Radios like to be up high. The problem, if there is one, is reaching too many cell towers at once, not lack of ability to reach a tower.