Because they had just won independence from Britain.
Explanation:
A lot of Americans feared that creating a strong federal government with too much authority over the states would only replace King George III with another tyrant.
People were afraid that a strong centeral gov't will take away the rights that the people had fought to gain. They were worried that it will be a new form of monarchy within the nation. People want the states to have more power so that the centeral government could not take away from the rest of the people.
