Neanderthals lived from 800000 years ago to 40000 years ago.
1.Climate change which produced changes in the hunting habits they relied on. They adjusted well to multiple ice ages and their body was adapted to harsh temperatures. They were dependent on hunting bison, deer, hairy rhinoceros and mammoths. Neanderthals made these species extinct which lead to their own demise.
2.They were absorbed by Homo Sapiens ,we carry some Neanderthal genes.
3.Inbreeding which lead to genetic diseases such as hemophilia and reduced fertility
4.Warfare with Homo Sapiens
5.Ilnesses brought by Homo Sapiens who was rezistant to
6.Natural Catastrophe such as volcano eruptions in Italy 40000 years ago
7.Competitive replacement by Homo Sapiens who were better hunters than Neanderthals havinf better planning skills due to better parietal lobes and took their food and developped agriculture
8.Poor social skills Homo Sapiens developped division of labour and Neanderthals did not adjust beeing good only for hunters
9.Reduced natality : they were producing less offsprings.Due to the large size of their skulls women haad trouble delivering babies.
10.Creationist theory : we did not find any complete Neanderthal skeleton but rather just parts here and there. Creationists claim that early hominids did not even exist and some jaws or fossils found could be from other apes
https://www.inverse.com/science/what-killed-the-neanderthals/amp,
https://en.wikipedia.org/wiki/Neanderthal_extinction,
https://news.stanford.edu/2019/11/07/new-theory-neanderthal-extinction/,