Rock maple has many advantages for guitar building - strength, stiffness, takes finish very well, is not a tropical hardwood and it sounds good. Some of Gibsons best sounding guitars have used maple extensively. The LP was originally to be maple body and neck - weight concerns prompted the change to mahogany.
With it's much slimmer body, the SG can use maple, yet not be too heavy. (If the SG design had come before the LP, I'm sure the all maple construction would have become standard).
Walnut is a good tonewood - maybe a bit on the dark side like mahogany, but brightened up a bit by the ebony fretboard on "The SG" models. It has a nice clean sound, but can scream with a good amp.