I assume that a big factor here is that the nitrogen in the air isn't "freely available" in the same sense that it is in nitrates in the soil. It's stuck to other nitrogen atoms, and N2 has a huge bond energy compared to the nitrogen-hydrogen bond in stuff like ammonium ions.(To digress, the fact that nitrogen atoms so desperately wants to bond with other nitrogen atoms is what makes nitrates so reactive in things like explosives. The formation of N2 releasing a lot of energy is the flip side of breaking N2 requiring a lot of energy.)

As to the reason why plants need nitrogen (the element as opposed to the gas) in the first place: it's a major component of chlorophyll. I mean, probably other reasons, but definitely that one.