Once the Abrahamic Religions (Christianity, Judaism, and Islam) became huge. It’s all about “Women are evil sluts who do nothing but sin and tempt the good and pious men”, it’s pretty evident in modern societies that have large populations of people that follow these religions.
Victorian England is responsible for most of our annoying modesty rules. As to why… I’m not certain, they were just fucking prudes.
Women have been covering their breasts for modesty for ~5,000 years, not 150. Used as a means of identifying which women are “respectable” and which women are “publicly available” to men, upper class women in ancient Mesopotamia were made to wear veils that were wrapped loosely over their hair and fell to their waists, but poorer women were not allowed, and faced harsh punishments if they dared to wear them.
It’s a conspiracy by Big Dairy to keep us udderly focused.