The Framers included the Supremacy Clause in the Constitution because they believed that the national government needed to have more power than the state governments had. They felt that the state governments had previously had too much power and that the country would be better off if the national government were more powerful.
First, the Framers felt that there would be no point in having a country if the federal government’s laws were not supreme. If the states could pass laws that would override the national laws, there would be no real sense in which the states were united. The Union would essentially just be a confederation in which all states were sovereign.
Second, the Framers believed that a confederal form of government was detrimental to the country’s economy. They felt that the states had been taking actions that created a poor economic climate. The states had been passing laws that did things like forgiving debts. They had been engaging in trade wars. Both of these things made it much harder for the country to thrive, they felt.
So, the Framers included the Supremacy Clause because they felt that it was necessary in order to have a strong central government that could create a positive economic environment.