What was one of the reasons the Anti-Federalists were opposed to the Constitution?

They afraid central government to overpower state government.

Explanation:

The Anti-federalist argued that a strong central government would make the state government become irrelevant, or losing state autonomy.

They also use Article 1 Section 8 Elastic Clause which provide Congress the power " to make any law that necessary and proper" to against ratification because it give an indefinite power to the federal government.

The Anti-Federalists opposed the ratification of the 1787 U. S. Constitution because they feared that the new national government would be too powerful and thus threaten individual liberties, given the absence of a bill of rights.