They didn't go completely to zero. Neanderthal genetics live on in us:
https://www.livescience.com/42933-humans-carry-20-percent-ne...
So at least some of the explanation is that they were absorbed into a larger population.
The Scottish Wildcat is a present day example of this:
https://en.wikipedia.org/wiki/Scottish_wildcat
One of the reasons it's going extinct as a separate species is because of interbreeding with feral and domestic cats.