I’m sure it has to do with the fact that christian belief correlated nudity with sex. Therefor when they were enacting the new social standards (sex now being bad, unless used for procreation) open nudity (sex) became something bad (shameful).
Thats why in America nude beaches are so taboo still. Christian idealism has overrun logical thought.
But that is just my general theory. The reason is all very relative to the specific culture.