Upvote:6
Part of the answer may rely in the fact that post-Western Roman Empire European nobles/royals were not descendants of older established noble families, but were often warlords from what Romans would have considered "barbarian" tribes. They grew up in tribal, sometimes semi-nomadic, environments where sewerage and running water were not known or considered "must-haves".
They didn't have it and didn't know they wanted it.
What I say would apply mostly to the beginning of the Middle Ages and they gradually adopted higher standards.