[29athrowaway]

Some think they are stupid but we know they had 1700 cubic centimeters of brain volume compared to 1200 cubic centimeters for an anatomically modern human.

It is possible that they were not that stupid and they were simply outbred by humans.

[inglor_cz]

They were certainly very conservative in their stone tool design. Mousterien tools change little over more than 100 000 years of their existence.

[29athrowaway]

They also lived in an ice covered world full of lethal megafauna instead of a post ice age world with fertile lands ready for agriculture and smaller animals.