Why use nitrogen in airguns?

Nitrogen is not used to power air weapons, I think you may be getting confused with CO2 which is a common air weapon propellant:

Correction. Nitrogen IS used to power airguns and has been for some time. Just look at the new "NITRO Piston" air systems produced for Crosman and Benjamin.

Nitro Systems, unlike spring systems, can be left cocked over a long period of time with no effect on the firing system they also do not cause spring wobble when fired. The gas is considered very stable.