That is the 64 thousand dollar question indeed. The Kurgan Invasion theory says that warlike hordes who worshipped a god of thunder rather than a life-giving goddess came down from the North and either overran or assimilated peaceful egalitarian cultures. The disruption spread simply because it was so disruptive.

https://en.wikipedia.org/wiki/Kurgan_hypothesis

"Gimbutas believed that the expansions of the Kurgan culture were a series of essentially-hostile military incursions in which a new warrior culture imposed itself on the peaceful, matrilinear, and matrifocal (but not matriarchal) cultures of "Old Europe" and replaced it with a patriarchal warrior society,[26] a process visible in the appearance of fortified settlements and hillforts and the graves of warrior-chieftains:"