Composition of air in an airliner tyre?
Nitrogen
i guess nitrogen is lighter, and it does not ignite when tire has a temp. and it's not flammable. and i guess the reason why they use nitrogen on center tanks now for most of the new boeing (cx 77w start with KPK ) & airbus have something call nitrogen generation system to fill the center tank with nitrogen to prevent sparks in the tanks. airbus has it own name on it but does the same purpose.
maybe google can give you a better answer on why they use Nitrogen on aircraft tires in case they ask you why.