I blame the uptight protestants who needed a severe moral code or something.

Also, for a time period, in some tropical places bare breasts supposedly meant a prostitute was working.

I think that in Korea, up to the early 20th century, women of age were topless to show they weren't married. Covered up meant they were no longer free? Need a source on it for specifics if anyone cares to search.

And it was in an African tribe i think that the opposite happened. Bare breasts meant a mother that was actively breastfeeding.

It's all foggy memories though, so accuracy is low.