I can surmise that the conflicts after the fall of rome made for a system like feudalism but how did it manage to get such a large amount of the population so accepting of roles little better than slaves?
Without meaning to discourage further dicussion, there is some interesting stuff in the FAQ about whether "feudalism" existed at all. See u/idjet's AMA here and u/Miles_Sine_Castrum's update on the state of the academic debate here.