taste test. They had most of the RootBeers on the list in their taste test.
People ranked the various ones they tried, and much to their surprise A&W ended up the winner... and it wasn't even really close. Many people thought they were even voting for their claimed favorite... when the reality was, A&W was what they liked best when all other factors were removed.
I think people assume as a "Brand Name" RootBeer, it won't be as good as the smaller Label brews...but there is a reason it has been as popular for as long as it has.