Due to the high temperatures often reached in any aircraft tires rapidly expansive gases are not suitable. Many elements in air are rapidly expansive and would cause blowouts on most landings. Nitrogen is less expansive at high temperatures therefore perfect for aircraft tires.
Nitrogen is also not an oxidizer. This means if the pressurized gas from a tire decides to blow out onto the grease and dirt on a strut, which is also right next to a hot brake rotor, it shouldn't help anything burst into flames.
Air2gxs & Minutman, you are both right. Also N2 is devoid of moisture where as compressed air is full of water. Water can collect inside of a wheel/tire and create an imbalance. An out of balance tire on your car is obnoxious at 65 mph, you can imagine the effect at 120mph.