I suspect the reason Directbet gets so many people rating them is because of their giveaways, free BTC (even if it's only 0.01 BTC or less) is a pretty effective way of creating goodwill.
To be honest thats could be the reason that most people voted them apart from their innovative feature that allow you to bet without having an account. Furthermore , I do think that they deserve a better rating than the others because apart from their generous giveaway ,
they got the best support ever that reply to your query within split of time. Most sportsbook' support here takes some time before addressing your query
P.S : for some people including me, the best of the services is judged from the how responsive their support is