I heard it had something to do with the visual centers of the brain being less active when things are dark in a dream, which makes it hard to make the dream suddenly brighter like you can when turning on a light. I've heard some people claim to be able to turn on lights, but most can't.

I suspect it has to do with the suddenness of the change, since most massive visual changes like that usually take at least a second. I find that I can make things brighter in dreams most of the time by staring at my hand until the detail and color appears, but lightswitches don't work for me either.