Why is alcohol restricted to 21 in the US, while other potentially harmful things are allowed at 18?

It seems rather odd to me that an 18 year old in the US is allowed to enter the military, be drafted, smoke cigarettes, take on loads of college debt, and buy a house (despite the unlikelihood of that), yet the legal drinking age is set at 21, even though human brain development ends at around 25. What are the reasons for this difference?