I've been inclined to think it has something to do with communication and community-forming tendencies. It seems that a number of other species, including Octopuses and African Grey Parrots, have pretty significant intelligence for animals. Yet their greater intelligence than most other animals doesn't seem to get them a really game-changing advantage over them. Human intelligence, or perhaps communication and organization in combination with them, has given us a series of game-changing advantages in a row, so many that our civilization is utterly dominant over any area where we choose to be.