You can pretty much blame MADD for that. Originally it started out as an organization with the best intentions but over time it's turned into more or less a modern temperance movement.
The key was figuring out that if you pressure the federal government enough you can get states to fall in line. The federal government doesn't have the ability to set a blanket drinking age for every state. But it does have the ability to withhold federal highway funds from states that refuse to set their minimum drinking age at 21.
The key was figuring out that if you pressure the federal government enough you can get states to fall in line. The federal government doesn't have the ability to set a blanket drinking age for every state. But it does have the ability to withhold federal highway funds from states that refuse to set their minimum drinking age at 21.