The Anti-Federalists favored strong state governments because states had led the American people to victory in the American Revolution.
all of the states had already passed bills of rights.
they believed that was the best way to protect people’s liberty
they believed states would more strongly resist an attempted takeover by Great Britain.

Democracy and darwinism would be my answer.

Assemblies do not have written codes of laws. laws are matters for magistrates and courts, not assemblies. codes of law are things such as civil law, criminal law, and constitutional law. what assemblies have are procedures regarding debating, voting and other business they might carry out.  also note that rome had three assemblies: the assembly of the soldiers, the assembly of the tribes, and the plebeian councils.