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.
That still doesn't explain the age of 21. Couldn't the federal government just as easily had required states set the age at 18 in order to recieve highway funds.
The age was already 18 in most states, so that wouldn't have done anything.
The 21 age was chosen partly because there was already historical precedent in holding back certain rights or privileges until the age of 21, such as voting prior to the 26th amendment. Also, it helps that it's only 3 years from 18, so it can be written off as just a minor change.
As I mentioned MADD basically transitioned into becoming a neo-prohibitionist/temperance movement organization. Even the founder, who left the group the year after MADD succeeded in getting the National Minimum Drinking Age Act passed, believes that to be the case. Shifting the drinking age is one part of that, they've also campaigned for increasing the excise taxes on alcohol to reduce consumption.
Yes, but MADD and other interests have pressured the federal government to set the age to 21 via highway funding, so states are constrained from deviating.
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.