Why did people ALWAYS wear hats until the late 1960s?

This question is regarding North American culture. I met a Muslim lady who told me that "covering a lady's hair/head" was a Christian belief to begin with, and that men and women always wore hats in history. Some hats told the job the man had or the tribe/country they were from, but for women it was always decency and modesty, even for the Pilgrims and Pioneers in the New America. Soon N. American's hats changed to something less "modesty" related, and more "fashion" related, but still, a man or woman would not think of leaving the house without their hat until the 1960s. Do you believe they always wore hats because of that old tradition of decency and modesty? Or just because everyone else wore hats? Do you have any ideas why they stopped during the 1960s? Do you think the fashion of men and women always wearing a hat will return?