There has long been speculation that Neanderthals were fully adapted to temperate latitude ice age Earth, even having a bony protrusion in their sinuses to help warm inhaled air. However, when the Earth warmed, these protrusions could very well have contributed to lethal chronic sinus infections.
Those who have had such infections, lasting six months or more, will testify as to their debilitating effects.
And just fleeing north would be no help, because there is a big difference between cold temperatures with a long growing season, in what are now the temperate latitudes, and the short growing seasons in the far north.
Periods of darkness lasted much longer back then, because there was far greater tilt to the Earth’s axis. Which explains why so few ice age creatures survived.
Good thing our Cro-Magnon ancestors invented the SUV to warm the earth.