I read somewhere that the prevalence of narrow victories is by design (or evolution maybe): 1. the court chooses what cases to hear and is unlikely to spend time on anything they all agree on; 2. the body of precedent is large enough so that lower courts aren't making decisions the court would overturn with a wide margin; 3. the parameters of a majority opinion are written to get 5 of 4, anything else is overkill or watered down.