Well modern humans was just lucky in the fact that they were able to "out adapt" Neanderthal.
Life is about luck.
I agree. Pure luck.
I heard somewhere before as well that homo sapiens killed off the neanderthal species... It makes sense and as porn$tar said, homo sapiens are inherently the more aggressive species.
But since this topic flared my interest, I did some research (ok internet research, I'm not a scientist!) and it's said that
Sourceneanderthals were a contemporary subspecies which incidentally bred with Homo sapiens and disappeared through absorption.
So now we have diseases, primitive warfare, cannibalism, cross-species dating and climate change. These are the possible causes of the neanderthal's extinction. Sounds very unlucky to me...