Originally Posted by Baying_Hound

From my years of paintball playing, this statement you have quoted is absolutely true. We used to use things called anti-siphon tanks to prevent liquid CO2 from entering the gun upon rapid firing. How does it happen? The universal gas law (PV = nRT) governs pressure, volume and temperature. Basically, as the pressure drops rapidly, so does the temperature. If you've used a CO2 inflater before you will notice as it fills the tire the cartridge becomes extremely cold. At these temperatures, liquid CO2 can exist at standard pressures (until it warms back up again of course).

To the OP, CO2 is cheap, compressible and easy to package in a small inflator cartridge. There is already a large market for CO2 in small packages already, so it is the logical way to go with inflators.