Couldn't it be as simple as as a selection of ample seed production over ample nitrogen availability in an environment where adequate sunlight was more often a

Couldn't it be as simple as as a selection of ample seed production over ample nitrogen availability in an environment where adequate sunlight was more often a limiting factor than adequate nitrogen. It takes significant energy to produce the root exudates that encourage and feed the specialized symbiotic bacteria that will fix some amount of nitrogen for the roots. The plant already needs lots of energy to build it's carbon structure and immune system. Then more energy for seed production, which is the most important of all. This energy has to come from sunlight. In growing competition with other taller plants and trees, sunlight is often greatly diminished. The plants that 'stopped' producing these exudates and used the available energy in structural fitness / robust seed production were able to compete more effectively as a species over a larger area.