Nitrogen gas really likes being a gas. You have to put a lot of energy into breaking it apart. Like lightning strike levels of energy. Indeed lightning strikes are responsible for turning a significant amount of nitrogen into ammonia. But, lightning is not the main source of usable nitrogen. Aside from human production of ammonia for fertilizer, bacteria are main fixers of nitrogen.

There are a family of enzymes (Nitrogenase) that can be used to reduce the amount of energy required to turn nitrogen gas into something useable by plants. However, this enzyme breaks down in the presence of oxygen. Unlike nitrogen, oxygen gas isn't particularly happy being a gas all itself and will react with nitrogen compounds quite readily (fertilizer is known for being explosive, and the N in TNT stands for nitrogen). Plants are heavily involved with oxygen, either producing it during the day or consuming it at night. If plants tried to produce this enzyme for themselves it would rapidly break down in the presence of the oxygen being produced by photosynthesis.

As such, nitrogen fixation is left to the bacterial specialists.