why are we here alone?

scientists speculate that 100,000 years ago our species was one of many. neanderthalensis, luzonensis, denisova may have been here with us. we were not alone. so why are we so very alone now? francis galton might have argued we were the most fit to survive, but what if we were simply the most brutal? and other hominid species fell in the wake of the onslaught of us.