Congress has the power of controlling funding they choose to hand out, and this doesn't meet the test of being "irresistibly coercive" to states. And states really want that highway funding...
Interesting case law the above poster is referencing I think: https://en.wikipedia.org/wiki/South_Dakota_v._Dole
Notably one of the dissenting judges said this: "establishment of a minimum drinking age of 21 is not sufficiently related to interstate highway construction to justify so conditioning funds appropriated for that purpose."
It did however pass the supreme court at 7-2.
Not necessarily. Pretty much every state had to ask themselves the same question: "Do we care more about highway funding or the very small number of people 18-21 who vote?"
It's actually a very easy question in political calculus terms.