I apologise if this is a very silly question, but why do pistols use CO2 or "soda-syphon" 12g cartidges instead of compressed air?

My line of thought has been prompted by stumbling across Steve's article on how to make re-fillable 12g CO2 cartridges. I do not have access to the necessary lathes etc he used in his article, so I doubt very much that I would be in a position to avail myself of the advantages he so abley demonstrates.

However, taking his process as an example, why could those same refillable cartridges not be filled with compressed air via a hills/FX stirrup pump or even a divers bottle. Does the CO2 contain properties that make it so much better than compressed air when used in a pistol?

Kind Regards
Bob