Why do they fill Airplane tires with Nitrogen?

Aircraft tires are filled with nitrogen because nitrogen gas is mostly inert, meaning that it requires more energy to react with other substances.

This is important because at elevated temperatures, oxygen can react with rubber.

Oxidized rubber is weaker than non-oxidized rubber, and weaker tires are not preferred.
Oxygen diffuses through rubber much faster than nitrogen.

This means that tires filled with 95% nitrogen (the standard percentage of nitrogen gas for inflation purposes) will maintain pressure longer. This leads to less maintenance, which lowers costs.