Those little CO2 cartridges were orginally used for soda water. You know those seltzer bottles, you probably last saw one in a Warner Bros cartoon, Bugs Bunny spraying Elmer down with a bottle with a lever on it. Opposite side of the lever is a little cartridge that gets unscrewed, the little tank gets put in, and it's screwed back together. Anyway, that's what the cartridges were designed for. Since they were made for that, many other uses have happened. There's a few homebrew beer kegging systems that use them, I've seen computer air sprayer thingies that use them, and of course, they're a ready source of compressed gas for things like bb guns and paintball guns and, yes, emergency bike inflators.

So the "Why CO2" is because there's already a market for it, and as cyclist we're probably soaking up a half percent of the total production. If we wanted air, or nitrogen, or some other reasonably inert gas (argon?) we'd have to pay for a custom production facility for 'em.

Thanks, I'll let the heavy drinkers pay for my speed inflators. Oh wait. I'm still paying for it. D'OH!