It’s entirely possible that Neanderthals didn’t just die out, but rather ceased being a distinct group due to genetic swamping. They interbred and blended into the larger group. How many Europeans today have some Neanderthal ancestry? It’s like the smaller tribes that ceased to exist in the southeast. Either they joined with larger tribes or intermarried with European settlers. Either way, their genetics live on in nominally white and black people, as well as within those other larger tribes.
“It’s entirely possible that Neanderthals didn’t just die out, but rather ceased being a distinct group due to genetic swamping. They interbred and blended into the larger group.”
Another possibility is that Neanderthals may have been very intelligent (even more intelligent that humans) but not very aggressive. When competing with homo sapiens they may have been wiped out intentionally or unintentionally. Note that all Europeans and even asians have some neanderthal genes, but not sub saharan africans. That may account for the disparity in intelligence.