The US Constitution only gives the Federal government the power to regulate international and interstate commerce (and of course other powers too, but those aren't relevant here). Article I section 8: https://www.archives.gov/founding-docs/constitution-transcri...
The Bill of Rights (first 10 amendments to the Constitution) explicitly states that all powers not explicitly granted to the Federal government are retained by the states and by the people. So that's probably why this limitation exists.
Of course, the Federal government has really stretched the meaning of "To regulate Commerce ... among the several states" and has gained quite a bit of power out of that line. But it's still a limitation.
The Bill of Rights (first 10 amendments to the Constitution) explicitly states that all powers not explicitly granted to the Federal government are retained by the states and by the people. So that's probably why this limitation exists.
Of course, the Federal government has really stretched the meaning of "To regulate Commerce ... among the several states" and has gained quite a bit of power out of that line. But it's still a limitation.