The reason, which is pretty unsupportable given that it's subjective, is that the blacker ebony is the more beautiful it is. A subjective proposition, but widespread. Hence the premium. Having said that, and having implied it's a rather shallow point of view, I should admit that I subscribe to it. I could excuse my own shallowness by offering the mitigation that the only ebony board I own happens to be the most playable, the smoothest, the most seductive I've ever encountered. And that'd be true. But the depthless blackness is a big factor in my affection for it.