The Puzzle of Intelligence

Human intelligence--including complex language, advanced tool use, and elaborate social organization--is clearly among the most powerful adaptations ever to evolve on planet earth. So why is it unique to the human species? One would think that such a powerful set of traits would be extremely useful to almost any species, yet no others have evolved these traits. Why? In other words, why have evolved brains always been so small compared to ours? Why, say, have our primate relatives not evolved our level of intelligence? Suggestions welcome.