Perhaps because it is simple.
You sign up, fill in the profile section, and they send you surveys that are easy to complete, and pay quite well. You can set your account up to get your prefered type of gift voucherwhen you reach the threshold, and it arrives in the mail. I love getting David Jones gift vouchers, it make my wife sooooooo happy.
If you haven't tried Valued Opinions yet, the you should. You can read how I rate them here.