Because the states were preexisting political entities, the U.S. Constitution did not need to define or explain federalism in any one section but it often mentions the rights and responsibilities of state governments and state officials in relation to the federal government. The federal government has certain express powers (also called enumerated powers) which are powers spelled out in the Constitution, including the right to levy taxes, declare war, and regulate interstate and foreign commerce. In addition, the Necessary and Proper Clause gives the federal government the implied power to pass any law "necessary and proper" for the execution of its express powers. Other powers—the reserved powers—are reserved to the people or the states. The power delegated to the federal government was significantly expanded by the Supreme Court decision in McCulloch v. Maryland (1819), amendments to the Constitution following the Civil War, and by some later amendments—as well as the overall claim of the Civil War, that the states were legally subject to the final dictates of the federal government.
Why did the constitution didn't have to explain federalism? 
Because the states were preexisting political entities