But I have an idea why we think there is more than one. Its because we are wrong about what is possible. Its because the model we carry around in our head of the world is quinzillions of times simpler than the actual real world.
So couldn't there possibly be a world where I posted this blog with a spelling error? Well within the super-duper over-simplification of the world we can think about, which has to fit inside a brain which is, again, one-quinzillionth the size of the actual universe, it sure SEEMS like that is possible. But that's just because we are overlooking like a quinzillion (give or take a trillion) facts about the actual universe, and probably a bazillion of those, if we knew them, would completely rule out a different past than the one we see.
Now of course, my mind isn't even beginning to be big enough to know for sure that there can only be one possible universe when all the facts are taken in to account. But following a nice idea from the blog I linked above, the multiple worlds hypotheses, as a class, are not the best explanation of anything. So in my economy of explanation, I'm not ever going to seriously use them. So I don't believe in them, q.e.d.