I suspect it has to do with phytonutrient in the skins, much like grapes and red wine made red by the dark grape skins. White wines can be made from red grapes if the skins are immediatetly removed -- sometimes appearing with a 'blush'.
Also, juicing increases the concentration of sugars taken in as the fruit pulp/fiber is jettisoned.
As an aside, I suspect black currants and elderberries are also very beneficial. I've seen studies indicating that but do not recall details at this moment. The dark skin berries seem to be gangbusters in the antioxidant, anticancer, anti-whatever environment.
Maybe if I eat enough they will turn my septic system into a pool of rose water? I know, I know, I really need to eat roses for that.