If there's any law to prevent total certainty, I'd guess it would have to do with the fact that we'd need to have a certain depth of understanding of the entire universe, and since we're in the universe and cannot fully understand ourselves, total certainty is impossible. And I'd agree with the idea about religion; people have a tendency to shy away from a state of doubt, and really until a decision is made about what we'll believe, we'll be in a state of doubt simply because there's no way to gain true certainty about such a subject. Though of course, the external security from having many people who believe as they do probably is a big part of why it's attractive too.