Why was the 10th amendment added to the bill of rights

The powers not delegated to the United States by the Constitution, nor prohibited by it to the States, are reserved to the States respectively, or to 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.

The Tenth Amendment was added to the Constitution of 1787 largely because of the intellectual influence and personal persistence of the Anti-Federalists and their allies. It's quite clear that the Tenth Amendment was written to emphasize the limited nature of the powers delegated to the federal government.