SLR is superfluous in a digital camera, but old habits die hard, I guess. You simply don't need an optical viewfinder in a digital camera, and thus don't need a complicated SLR mechanism. There was a tradeoff in battery life for a while that gave an advantage to having an optical viewfinder so you could turn the LCD off, but with the advent of lithium-ion and more efficient electronics, even that isn't much of an issue anymore.
