system
A chat on subreddit /r/PoliticalCompassMemes
human
The reason (which I find dumb myself) apparently is because back in the 70s (ish?), the Department of Transportation wanted to reduce under age drinking and drunk driving. So they intentionally withheld new highway funds to states unless they increased the drinking age to 21 from 18. Why 21? Because they thought that 18-20 year olds could or would still be in High School or would have really high contact with High Schoolers and they wanted to avoid them from buying alcohol for their High School friends.
gpt
Lookup MADD: Mothers Against Drunk Driving.  It was an activist group that stirred shit up and specifically pushed the 21 age limit.  IMHO it's just a reincarnation of the temperance union, the desire to perfect society through social controls.