There is extensive research that support the theory that alcohol effects development of essential parts of your brain, which finishes developing around 21 at average. Not to mention we know adults make stupid enough decisions drink alcohol there's no need to allow teens to drink as well.
P.s. I'm not against teen drinking completely, my parents allowed me to drink because I was responsible and did it very little, but it definitely shouldn't be unregulated like it is in the countries outside the U.S.