2. Registering firearms isn't completely effective: Because everyone knows that if you have to use a firearm for something illegal or at least questionable, you make sure that the firearm found by authorities isn't your firearm, or at least, one that's not registered to you. And that usually means stealing it or smuggling it in.
3. Jobs: Let's face it. Sooner or later, if someone wants to get a certain thing, like the latest electronics or a new fridge, they're going to prefer one made in their country. The same would apply to firearms.