I can't speak for any of the states, but I know when some of the provinces in Canada were debating the drinking age, most raised it from 18 to 19. It's not that they don't think 18 year olds should drink, but that 16 year olds can pass for 18 year olds. And, as some 18 year olds are still in high school, there became a possibility that they would become providers of alcohol for their younger peers and juniors. I expect that there is similar reasoning behind the U.S. choosing 21 as a drinking age.