Here's a case where the allegedly pro-gun Republicans decline to support gun rights. Also, the notion that national prohibitions might work because local ones are ineffective is doubly flawed. First, drug prohibition is national - and a dismal failure. Second, whether guns are prohibited locally or nationally, the Second Amendment forbids this prohibition. Some people claim that "only the federal hand is stayed by the Second Amendment" - in which case only local prohibitions would possibly be legal. Of course, others claim the Second Amendment is incorporated by the Fourteenth, so that even local prohibitions are also illegal.
The latter was the explicitly stated intent of the drafters of the Fourteenth Amendment. Various courts (most notably in the Slaughterhouse cases) slaughtered the English language to weasel around the amendment.
Basically, it was a political decision -- the Feds simply gave up on trying to make the defeated rebels treat freedmen as equal citizens as part of the bargain for accepting the heist of the 1876 election, and the courts went along.