Because it runs counter to the narrative of gun control advocates. Almost no gun crime is committed with legally obtained guns by previously law-abiding people.

Because it runs counter to the narrative of gun control advocates. Almost no gun crime is committed with legally obtained guns by previously law-abiding people. The pro-2A response to gun control requests based on crime is often to say "don't ban my guns - just make 'felon in possession of a gun' a life-without-parole offense".