it allows states to handle their own sensitive social issues and keep the role of the federal government limited only to things that can only be handled on an federal level, especially if there are no travel and trade restrictions between the states of the union.

But the people controlling state issues are elected in those states (Governors, State Senates, Town Councils, Mayors, and so on). If the Federal Government is limited to only things at the Federal level, yet the people who are doing those things are elected at the state level, why do they only act on behalf of their constituent if their actions, bills they introduce and pass, issues they speak about are national ones?

I understand the concept of representation and the republic of states, but it just seems hard to reconcile the fact that national issues are in their purview yet they only "represent their constituents". Congresspeople aren't only introducing bills that affect their districts alone. Not that they don't do things in their own districts, but they also are part of the Federal Government and a lot of them don't seem to take that responsibility.