Because according to Madison's writings, that is outside the scope of the commerce power that was given to the federal government.
Just the opposite. Madison said Congress could do that with foreign nations, didn't he? He said "regulate commerce with foreign Nations" (from 1-8-3) gave Congress that power.
But you're telling me that, in the same Section of the Constitution, "regulate commerce among the several states" means something completely different?
OK. Where's the proof of that?