Plants must have nitrogen to grow. According to the answer to this question, there are no plants that can fix their own nitrogen (without the help of bacteria).
Plants get their nitrogen in the form of nitrates (NO3-) or ammonia (NH4+). Nitrates and ammonia get into the soil through
- lightning (causing N2 and O2 to combine to form NO which then reacts with atmospheric water and is brought to earth by rain)
- nitrogen-fixing bacteria associated with leguminous plants
- fixed nitrogen from dead plants/animals (ammonification), urine, etc.
- ammonifying and other bacteria
The answer also states that it seems to be possible to engineer plants to be able to fix nitrogen.
So one could think plants could have evolved to be able to fix nitrogen, but did not. Maybe it just did not happen based on it's probability to happen.
Is there any other explanation?
Maybe the ability to fix nitrogen would have a harmful effect on plants? Or fixation requires a complex set of related mutations? Are there enough areas with soil depleted of nitrates to exert a selection pressure for nitrogen-fixing plants?