The kind of interesting question and there's been quite a few potentially interesting answers

The fundamental reason that we are the only species of our genus remaining is that we murdered and or interbreed with all the rest of them.

Some of them just couldn't compete. And some of them we basically made friends with enough of them to mix together and kill the rest. Primitive homo sapiens were not a sharing species with our cousins. At least as much as I understand early human anthropology, which is admittedly not well.