Traditional air-guns have always been gas powered, and this limitation was carried over with the invention of airsoft guns. Even with lighter and cheaper plastic pellets, compressed gas guns still have some limitations, chief of which is that they do not operate very well in low temperatures. Most airsoft guns using compressed air, can only shoot two pellets if it’s really cold.
The solution to this problem was with the use of electrical mechanisms inside the gun to compress the air. With the electric airsoft guns, the only real limitation would be the battery used. There’s a compromise between having a battery which delivers more muzzle velocity (faster pellet shots) and more more shots per battery charge.
The use of the electric airsoft gun, means that the gun is practically self-contained and that the user would only have to worry about recharging the battery after every use, instead of having the gas canister refilled. Charging batteries does come out cheaper in the longer run.