The states have rights too

The Tenth Amendment restates the Constitution's principle of federalism by providing that powers not granted to the federal government nor prohibited to the states by the Constitution of the United States are reserved to the states or the people. The Tenth Amendment was included in the Bill of Rights to further define the balance of power between the federal government and the states. Any power not listed, says the Tenth Amendment, is left to the states or the people. Although the Tenth Amendment does not specify what these “powers” may be, the U.S. Supreme Court has ruled that laws affecting family relations (such as marriage, divorce, and adoption), commerce that occurs within a state’s own borders, and local law enforcement activities, are among those specifically reserved to the states or the people. This amendment is around because we don't want our government to have complete control over us and this amendment allows our states to have some sort of sovereignty. States don't lower the drinking age because they're afraid of losing contact with the federal government which it has threaten to do to states who disobey the universal drinking age law.