It is hard to police guns when there is free travel between the US states, yet only individual states can be relied upon to pass any reform. A broken federal government means guns are easily exported from red states with practically zero gun laws to blue states where they are used to commit crimes. States are often forced to recognize rights granted by other states because such an interstate jurisdictional question naturally bubbles up to the aforementioned dysfunctional federal system.

Similarly to how many (most?) guns used criminally in Mexico actually come from the United States.

Edit: I'm not surprised by the downvotes, but I am amused. These are objective facts. Any basic research will yield many studies (including from the American government) showing that the majority of guns used in crimes in Mexico are traced back to the States. Americans love the boogeyman of dangerous Mexican cartels so much they never seem to ask themselves where these guns come from in the first place. Hint: look in the mirror.