Follow the money
A State can have an 18 year old drinking age, you just have to give up a whole lot of Federal highway fund money to do it
The National Minimum Drinking Age Act of 1984 required all states to raise their minimum purchase and public possession of alcohol age to 21. States that did not comply faced a reduction in highway funds under the Federal Highway Aid Act.
Much like NH does not have a seat belt law for those 18 or over, but it costs them Federal dollars to do so, at least 5 Million a year