It's tied up with the lack of good public transportation in most of the US. Being able to drink in Europe at a low age makes more sense, since drunken 16 year olds aren't getting behind the wheel of the car; they're just climbing into a bus or train to get home.
Unilaterally lowering the drinking age in the US without simultaneously building out public transportation -- expensive, given how US cities tend to be built around the assumption of cars and suburbs -- would be problematic, to say the least.