Main reason why CO2 is used is the FRA rules. We are not allowed to go over 1000psi/70 bar for pneumatics.
CO2 or air, all the rest of the inert gasses like Nitrogen ain't allowed since a few years. This to avoid event organisers having to set up a tankpark with any kind of inert gas available. Even with people bringing their own refill bottle.

A tad of further explanation.

Stored CO2 is expressed in mass (kg), as most other gasses are expressed in volume (liters).

1 Kg of CO2 is 512 liters of gas at room temperature and pressure. A 2 kg CO2 bottle has a volume of 3 liters. Therefore, there is 1024 liters of gas in that bottle.

With nitrogen/air/helium that same bottle can hold 210 liters of gas @70 bar. And even @200 bar it's "only" 600 liters.

I believe CO2 wins in storage capacity...