Ok so I figure I should get this right since I started it with erroneous info, so I did a bit of research. Nitrogen offers the advantage of being dry, inert (does not support combustion), and does not damage the tire inner liner. It is also stable with temp change which is a big concern in aviation. Not in a cessna as that is not much different than a car, but in airliners, fast jets, heavy transport.
Here's a couple of links with good info.
As for my comment about expansion at altitude I have no idea why I said that. Even though there is a difference in pressure at altitude the tires internal volume of gas will not change, regardless of the gas inside. The pressure differential would increase and the tire would 'grow' as altitude increases but it wouldn't matter what gas is inside as to how much growth you would have. Due to the construction of the tire it would probably be negligible.