Neanderthals WERE human! Just a different species of human. Exactly why they disappeared is not well understood. Many assumed modern humans killed them all off, except that most modern humans (Caucasians, and Orientals) typically have around 4% of the DNA coming from Neanderthals, so, clearly they did a lot more than wiping them out! They tended to live in very small bands, whereas modern humans often live in larger groups. This may have influenced survival rates, but, no-one really knows for sure.