This is a question that anthropologists have studied for quite some time. Homo sapiens were not always the lone species of humans on the planet. In fact, as early as two hundred thousand years ago, there were as many as eight species of humans. This all begs the question, why is there only one of us today?
Some anthropologists believe that our habits of behavior are partly responsible. It seems that Homo sapiens were more social than other human species. We lived in larger groups, making us less susceptible to inbreeding, which would weaken our genetic health. Also, by living in larger groups, Homo sapiens were able to build stronger communities. In tough times, larger communities meant that labor and tasks, such as gathering food and childrearing, could be better divided among more community members. Larger communities also meant that Homo sapiens were better able to compete for territory.
Homo sapiens were also able to migrate into a vast variety of landscapes. We are an adaptable species. It seems that we are more adaptable than some of our prehistoric counterparts. It appears that Denisovans, for instance, had a rather small geographic range. We do not know for sure why they died out, but it is possible that some localized catastrophe or a changing climate may have spelled their end. Homo sapiens were widespread rather early on. Therefore, localized events would not threaten the entire species.
This also seems to be the case with Neanderthals. They had lived in the forests of Europe long before Homo sapiens arrived there. However, when a changing climate during the ice age reduced the forests, they struggled to adapt. Homo sapiens, on the other hand, were accustomed to hunting in open landscapes. As a result, they were better positioned to survive the changing climate.