The expectation is that there are many areas of law and policy where the various individual states cannot effectively keep control. In such areas, federal control might be needed.
For example, with marijuana, state or local governance might not be sufficient. If marijuana is legal in one state and illegal in another, it would be too easy for the drug to be taken across state lines into the state where it was illegal. Because there are policy areas like this where actions in one state impact conditions in another state, there is a need for federal control.
We expect the government to be able to exert control over issues like marijuana (or like air pollution) that cross state lines. Therefore, we expect the federal government to deal with these issues since the state and local governments are not capable of dealing with them.