because those neat little air bottles go off like a heafty grenade if they were to get punctured. Your paying for the safety element.
Air is less efficient, however its kinder to the seals in all markers, and that means less wear and tear.
It isnt effected by temperature in the same way co2 is.
it has a more consistent output because all the contents of the bottle is in a gaseous form, not Liquid AND gas like in CO2.
In short, CO2 can be a pain in the arse, especially if its cold.
Air is more reliable, cleaner, and consistent, but more expensive too.
A good air system is worth its weight in gold