Here's why:

* It doesn't readily react with oxygen. Combustion requires a fuel source (something that can burn) and an oxidizer (usually oxygen). Nitrogen is very unreactive and doesn't participate in combustion reactions.

* It acts as an inert gas. In many situations, nitrogen is used to displace oxygen and prevent fires, because it doesn't support combustion.

However, it's important to note:

* Nitrogen can support some specific reactions under extreme conditions. For example, it can react with certain metals at very high temperatures.

* Nitrogen compounds can be combustible. For example, ammonia (NH3) is flammable.

Overall, nitrogen itself is considered a non-combustible gas and is often used in situations where preventing fires is crucial.