Back in the sixties all you heard was how it was "only Fair" to lower the age for drinking and everything else to eighteen. When that same generation started having kids they didn't want driving drunk and raising Hell the same way they did, Bada Bing, Bada Boom, pander, pander, it was all about raising the drinking age and instituting harsh penalties for driving under the influence.
It was pretty hard to argue that an 18 year old was responsible enough to carry a machine gun around rice paddies but not enough to have a beer.