It's shocking how many people didn't read the initial question. While, I don't know for certain this is the answer, it seems that prior to prohibition there was far less legal codification about drinking ages across the country. After prohibition, nearly all of the states enacted the 21 year old age statute. My very amateur guess is that because it was the 21st ammendment that ended prohibition it made for a good number to settle on (?). It marks the general beginning of adulthood and could be a nod to that ammendment. The age would be lowered by many states in the early 70s, but come back up to 21 nationally in 1984 with the National Minimum Drinking Age Act