A couple of studies from the medical school at the University of San Juan, Puerto Rico, shed some light on this. They decided to test the anti-cancer capabilities of a combination of the polyphenols from red wine, in concentrations equivalent to what is achieved by healthy drinking, against higher concentrations of resveratrol. The study used breast cancer cells in culture, along with a variant of the type that metastasizes to bone. Incredibly, the combination was far more effective, suggesting that the natural wine compounds working together, that is the key.
Ther are a number of explanations for this. One would be that there is a natural synergy between the chemicals that is not completely understood but seems to amplify the effect. Another is that these antioxidant molecules, for all their potency, may break down when they are extracted out of their natural context in wine. If you ask me, it is the theory that supplements are somehow better than whole foods (this includes wine) that breaks down under scrutiny. So while resveratrol as a pharmacologically stabilized compound administered under standardized conditions may well prove to be the next breakthrough in cancer prevention and treatment, that is not the same as popping pills from the health food store.