nitrogen in tires Why?

No moisture. Same reason you don't have spit into a sink at the dentists office anymore.

Compressed air has lots of water in it. That water can cause corrosion between the tire and wheel assembly which can lead to leaks. It also causes pressure fluctuations with temperature change. Nitrogen fill has virtually no moisture at all in it. In fact, I used nitrogen to purge the moisture from my portable air tank without having to remove the valve assembly. Less chance of corrosion related leaks, more stable pressures.

More of a gimmick. Pure nitrogen is supposed to be less prone to expansion and contraction by changes in temperature. Pure nitrogen is supposed to be more dense than regular air and is supposed to leak less.

I do know that regular air will expand and contract with temperature changes. My car has tire pressure monitors. I regularly see tire pressure increase about 4 to 5 PSI as the tires reach normal operating temperature while driving.

Yes, normal air is 78% nitrogen. But it is the other 22% that causes the che changes.

Personally, I just check my tire pressure every so often and don't use nitrogen. If checked regularly, you get an idea of how often to adjust the air pressure.

Lacking the o2, rust would be eliminated. Nitrogen is not influenced the same way by heat so tire pressure would be stable since the air inside would not expand.

More stable as mentioned, doesn't leak down as fast because it has bigger molecules, but the best and only reason to my mind, the lack of oxygen inside the rim absolutely stops corrosion of the rim. This is a big deal if you have chrome rims because they tend to corrode and leak inside.