The Framers of the Constitution chose federalism because it was an intermediate form of government that was not as centralized as the British government but more centralized than the government under the Articles of Confederation.
When the British ruled the American colonies, they had a unified system. The government in London ruled the colonies and could veto any laws that colonial assemblies created. The colonists felt that the government was too centralized. They felt that the government in London was too far away, did not understand them, and did not care about them the way their local governments did. Therefore, when they became independent, they wanted the exact opposite form of government.
The opposite of the British system was the confederal system that the US had when it first became independent. This system of government gave essentially all the power to the states. This meant that the national government had practically no power. This soon proved to be a real problem because the national government was so weak that it was essentially not a government at all.
Because of this, the Framers went to an intermediate form of government. They created a federal system where the states had some powers and the national government had other powers. That way, the government was “just right” after having been too centralized and then too decentralized.