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.
I dont think that many people give rate to Directbet just because the giveaways. I my self rate them at 5, because of its simplicity. No need to create account, instant bet and instant withdrawal. So I think those are the most possible reasons why people give a good rate to them.
But of course its all about "favorite", we all have our own favorite place to bet.