[hodgesrm]

One of the drivers for feudalism was the prevalence of cavalry in warfare starting after about 700AD. Mounted knights are expensive to maintain, hence the solution of providing land grants in exchange for military service. It's a medieval version of the military-industrial complex.

The connection between mounted warfare and feudalism is controversial [1] but it's common enough that societies adopt social structures optimized for defense. There were almost certainly multiple reasons for the prevalence of feudal relations.

[1] https://en.wikipedia.org/wiki/Great_Stirrup_Controversy