Why CO2 ?
CO2 is a good choice for making fizzy drinks, because not only does it dissolve in a liquid, like other gases, but also reacts with water to form carbonic acid, and its ions. Thus the equilibrium concentration of CO2 in the air ,vs solution, lies towards more total carbonates in solution than with a ideal, nonreactive gas. If you charged a bottle of Soda with helium, rather than CO2, most of the gas would come out in one great bump immediately upon opening. However, the kinetics of CO2 <->HCO3- <->CO32- are slow (~seconds), so not only do you end up with a lot more potential gas dissolved when you use CO2, but it comes out more slowly.