Nitrogen in the air is in the form of N2 and it rather stable because it has a triple bond between the N atoms. It takes more energy to break the N2 triple bond and bacteria have been selected to do that energy requiring step. It gets broken down in the soil by bacteria in to amino-compounds which are more easily reactive and accessible for the plant. It's all governed by thermodynamic. For the plant, it is easier to take up N from the soil because it is usually broken in to molecules that are only single bonded to it and N has an extra pair of unpaired electrons that plants access much more easily and with a favorable thermodynamic.