you should scan the book i mention. it makes a convincing argument that the anatomical differences are significant, starting with the daily caloric requirement, roughly 2x in neanderthals. anatomy better suited to ambush hunting , not running game down. poor strategy in open ground.
also, why do you say the didn’t die out? they became extinct. is there a difference?
Because the Neandertal DNA is found in living people. The book you mention is just another “conclusion to fit the assumptions” speculation and waste of everyone’s time.