You know how cling wrap sticks to itself really well? And if you pull out a lot of cling wrap, it is almost impossible to pull all of it apart at the same time? The Nitrogen in air (N2) is similar to that. It sticks together very strongly. Nitrogen sticks to itself about 3 times stronger than it sticks to a single Hydrogen. It takes a lot of energy to break apart N2, so plants instead get the Nitrogen from sources where it is bonded more weakly with other atoms, like nitrates (bonded to Oxygen) and ammonia (bonded to Hydrogen).