Good question, but the simple answer is that by chance, some genera (our genus is Homo) happen to have only one living species, and that's true for us. At other periods in Earth history, there were multiple species of humans living on the planet at the same time. As recently as 30,000 years ago (which is an absolutely tiny amount of time from the perspective of Earth history, there were three human species around: H. sapiens, H. denisova, and H. neanderthalensis. Why the others went extinct, we don't know, but it could have been competition (with us), chance events (like a disease that wiped out a then smallish population), climate change, or some combo thereof.
As an aside - are we that smart? We existed pretty much like any other animal, despite being 100% anatomically modern, for the vast majority of our species' history (200,000ish years). I'd argue that really the one thing that separates us so much from other critters is the evolutionary advancements in our vocal apparatus (as compared to other primates) and the eventual creation of writing, which has allowed us to accumulate knowledge from generation to generation. If each new generation of human couldn't be born into the wealth of pre-recorded knowledge from all prior generations, I doubt we'd look nearly so advanced.