Because evolution isn't actually guided and is really just a process of mutation, reassembly (for sexual species), then checking to see if it works well enough to pass genes on. Apparently red leaves work for that plant

We have diverse geography, and genetic diversity. Obviously this plant worked out a way to use the high energy of the wide green ranhe, without overcooking itself. But if you compare all plamt leaves and grasses of plant life green is the most common.