Air offers more pressure stability at our range of operating temperatures than CO2.
At any single temperature point, CO2 turns to gas at a very precise pressure and negates the need for a regulator on a bottle. Compressed air always drops in pressure as you use it. Even if you regulate the bottle, there will be a tiny amount of pressure drop at the regulator output as the pressure in the tank drops.
The problem lies when you pass CO2 through passages that cause compression and expansion. It causes pressure changes and results in temperature variations that can cause condensation, which in turn affects the end value of the pressure and makes it unpredictable when used in paintball guns, especially when fired in an erratic manner. The condensation also affects how the orings work. The temperature change during the day or a game also causes a change in the pressure value where the CO2 turns to a gas. This is where air has a distinct advantage of stability. The range of temperature change the air goes through when flowing through a valve or while sitting in the sun doesn't have much of an effect on the pressure. It will affect it some, but not to the same effect as with CO2, plus the dew point for CO2 can be reached easily where as for air it cannot.
Except for the Automag in front, its usually the man behind the equipment that counts.