[Slawomir (Suavi)]

As far as I know Argon has lower thermal conductivity than air or Nitrogen, so it also helps in keeping the sensor colder (can reach a lower sensor temp with Argon) and perhaps to some extent Argon's lower thermal conductivity stops the cover window from fogging up. Nitrogen has very similar thermal conductivity as air, since air is mostly nitrogen anyway. Xenon has even lower thermal conductivity than Argon, but it is not commonly used perhaps due to a higher price or some other properties that I'm not aware of.

EDIT: just checked and FLI also uses Argon in their cameras, and offers Xenon as an option (extra cost).