Airsoft guns were created with a specific need in mind: for a legal toy gun, which is adults can use in simulated combat. As such, these guns were designed to be non-lethal versions or exact replicas of actual military guns. Using plastic pellets, some of the earlier models of airsoft guns made use of existing gas technologies. Being an off the shelf, tried and tested technology, carbon dioxide (CO2) pressurized gas was a best choice in some models.
There are still CO2 airsoft guns on the market, however these are considered as entry-level or low-power guns which are not usually used in competition. Higher muzzle velocities can be attained with more advanced gases and other technologies. Additionally, more pellets shot can be shot with these competing technologies, and during competition that is a big advantage for the participants.
Of course, for an owner who just wants to get their hands on an entry-level gun, a carbon-dioxide airsoft gun is a good way to start.