Those who want to use a shield care about their AC more than damage. To use great weapon master is to not use the shield. Shield users may not be doing as much damage as great weapon master users, but they can do decent enough it's worth the trade for them for the +2 AC. Therefore, great weapon master is not better because it goes against their whole reason for wanting to use a shield. Whether it's better to use great weapon master over shield use overall as a game concept, that preference is up to the individual's opinion not objective fact.