Why are some powers denied only to the states?

Some powers are denied to states to maintain a uniform national policy and ensure the supremacy of federal law, as outlined in the U.S. Constitution. This prevents states from enacting laws that could undermine national interests or violate individual rights. For example, states cannot enter into treaties or coin money, as these powers are reserved for the federal government to maintain national cohesion and economic stability. Such restrictions help to create a balanced distribution of power between state and federal governments.