I'm not sure that's the entirely the case, given that, when I was a kid, you couldn't buy non-distilled beverages at the grocery store either.
https://en.wikipedia.org/wiki/Alcoholic_beverage_control_sta...
https://en.wikipedia.org/wiki/Alcoholic_beverage_control_sta...
I believe there's some relation between this and federal highway funding being tied to a minimum drinking age of 21, but I may be misremembering some half-read magazine article.