There were just too many humans for Neanderthals to survive
For about 300,000 years, Europe was pretty much the undisputed
domain of the Neanderthals. About 60,000 years ago, modern
humans entered the continent, and by 40,000 years ago, the
Neanderthals were gone forever. It's difficult to figure out
just what caused this extinction - and it's almost certain that
multiple factors were in play - but Cambridge researchers now
think they've hit upon the primary reason why Neanderthals died
out: there simply weren't enough of them to compete.  According
to complex statistical analysis of the so-called "Perigord"
region of southwestern France - which has the highest
concentration of early human and Neanderthal sites in all of
Europe - the earliest human immigrants to the continent
instantly outnumbered the Neanderthals by a factor of 10 to 1.
The basis for this conclusion is the massive uptick in the
number of human sites, the fact that these sites were more
densely populated than their Neanderthal counterparts