Let me tell you something interesting, real quick and to the point. There are 3 types of people, negroid, caucasoid, and mongoloid. Some 30 million years ago, there were the Human, Neanderthal, and Denisovan.
...The Denisovan looked like the Asian, the human looked like the African, and the Neanderthal looked like the caucasian.
...But the weirdest part of it all, is that the human reverted back to the original 3, because the light skin of the caucasian developed over time, and is NOT in any way related to neanderthal. It's completely a separate adaptation. It literally took 100,000 years after the end of the light skinned Neanderthal, for the new light skinned human to come into existence.
Neanderthals were humans. They interbred with the cro magnon people, and their DNA can be found in human populations today.
Most likely, the change in climate to a warmer environment made it difficult for them to survive. Warmer air holds less oxygen, and their thick muscular bodies required a lot of oxygen.
A perfectly adapted body for extreme cold became a handicap in warming climes.
They were not stupid, they had a larger brain than we do today, and contrary to past ideas, they most likely could speak.