The process you are talking about is called nitrogen fixation. It is done by the nitrogenase enzyme. Plants do not have this enzyme but live in symbiose with bacteria living on the roots and in the soil which does the nitrogen fixation. So there is normally no need for plants to do this themselves.

The problem is that nitrogen fixation takes a lot of energy. Unlike carbon dioxide the dinitrogen in the air require much more energy to break the bonds. So nitrogen fixating bacteria consume a lot of energy, much more then the leaf cells on a plant generate from the sun. These bacteria get their energy mostly from sugars released by the plants through their roots. When the plants lack nitrogen compounds they release their sugar straight into the soil and the nitrogen fixating bacteria use this sugar to make amonia and other nitrogen fertilizers. Nematodes and amoeba will then eat the bacteria and release any excess nitrogen compounds which the plants then absorb through their roots.

However by making nitrogen fertilizers from energy rich natural gas or electricity we are essentially doing the bacterias jobs for them. By adding this nitrogen to the soil the plants are never low on nitrogen and therefore does not release sugar into the soil and the nitrogen fixating bacteria dies out. This of course means that the plants are able to use the sugars themselves to grow faster and larger.