How about this: it takes energy to build a photo-synthesis machine. A machine like this won't consume all photons but only some range of electromagnetic spectrum. This is because you need to have such structure to allow light to pass through the outer part of the body of the organism, but be absorbed at a very specific place inside. This is why multi-junction solar cells exist, to use more of the solar spectrum. It would cost a plant more energy to build such a multi-layered chloroplast, so perhaps having just a single layer using a single range of the spectrum is the most optimal (at least as a local maximum).