one problem is that it is technically still a state decision. However, the federal government tied access to highway funds to states with a minimum age of 21. The linkage survived court challenges at the time, and no state could afford its highway system without federal assistance. Thus, we have a national standard dictated by federal control of the highway trust fund.
Then it REALLY is NOT a state decision, is it?
I'm not in favor of the way the federal government passed down this mandate. But the method was clever since it allowed the circumvention of state control over alcohol, which otherwise is untouchable with the 21st amendment.
The state decision is whether to set their own minimum drinking age, or to augment their road funds with federal money. Needless to say, every state that was not at 21 opted for the latter.