Consider the war on drugs- not just a complete failure, but actively harmful, especially to minority communities, while rich white users continue about their business largely unmolested. You can get anything you want your first time in a visit to any American city inside of an hour. Just stop by a restaurant kitchen. Banning firearms would be about as successful. So the argument is, why criminalize a huge number of people who aren't criminals just because some criminals and mentally ill people use guns? It would lead to another drug war situation.