Becuase guns have higher protections, more stringent seller regulations, and advocacy groups. In some localities, things like swords can be illegal to own. The dealers generally don't need a special license. With guns, you have an FFL which is heavily regulated. So as a payment processor, there's a greater chance that a merchant selling a sword might be violating the law than an FFL selling a gun. Then the advocacy groups for guns are much more active than the ones for knives and swords.
