Reggie

Air we breath is made up of oxygen, nitrogen, and water vapour plus other containments like ozone (bad for rubber).

Oxygen is more permeable than nitrogen - so it will not escape as easily - meaning you will maintain your pressure longer.

Mass of nitrogen is better heat absorption - needed for brake and tire friction.

Just for your information - aircraft tires have to use nitrogen because of the risk of the moisture vapour freezing in the tires as well as the tire see more extreme conditions than car racing.