I now the A12 and sr 71 had nitro in tires, but thats extreme. what are the beginning factors that dictate what is put in the tires?
Transport airplanes use dry (completely dehumidified) nitrogen from high pressure bottles for tire servicing. Shop air is "dehumidified" using water separators but will still have water vapour in it. Bottled dry nitrogen is completely moisture free.
Besides humidity, the main objective is removal of oxygen. Oxygen, the other major element in air at 21%, tends to oxidize the rubber, degrading it.
So using dry nitrogen eliminates humidity related problems and eliminates internal oxidation of the rubber (as well as making the inside atmosphere completely incombustible), so it's generally the standard for commercial aircraft where the tires run at extreme pressures and speeds.
Light airplanes will use dry nitrogen if it's available and the owner wants to go to the expense, or just regular shop air as you would a car.
Nitrogen has a lot of benefits when used in tyres, not just in aircraft but in cars, trucks, motorbikes and so on. First consider the alternative. If you fill the tyre with air you will get some water vapour, exactly how much depends on the humidity at the time. Water density varies between 0.6Kg/m3 to 997Kg/m3 over just 100 degrees C. That results in a big change of pressure from even a small amount of water. Air has many other things in it as well, including oxygen which supports combustion and hydrogen which leaks out reducing pressure. Nitrogen is cheap and it is boring. The molecules are all the same size so there are no leaks, it doesn't burn in an accident and the pressure remains linear with temperature. If you have the option, and most places that do a lot with tyres do have the option, then using nitrogen is a cheap and effective solution for managing tyres.