Air has a certain moisture content and it is generally very hard to remove this moisture. If an airplane tires were filled with air, at the flight altitude ice would form inside the tires since the temp up there is about -55 degrees C. Landing with a chunk of ice in the tire would make it out of balance and change the tire pressure. Tires would probably burst.
On the other hand, nitrogen doesn't form a liquid till -173C and pure nitrogen has almost no moisture.
In addition, consider if the brakes overheat and cause a fire. The nitrogen will not burn, but air has oxygen which will feed the fire. Jet airline tires are fused. When the fuse is heated it deflates the tire so they don't explode.
. .The huge tires on aircraft are expensive, and difficult to mount and dismount. They are also subjected to tremendous forces on landing when they must accelerate very quickly. The friction on touch-down creates great heat within the tires and produces very high stresses in the walls of the carcass.
Therefore, every effort is made to reduce or eliminate the deterioration caused over time by oxidation.
Since normal atmospheric air is approximately 20% oxygen, the tires are inflated with 100% nitrogen - a relatively inert gas.