Probably because they're wholly inequivalent.

Guns certainly have an entire range of illegitimate uses. Almost of of those uses are criminal. They also have a large number of completely legitimate uses, like hunting and sport. They also have a wide range of absolutely vital uses, like self defense, the defense of others, and holding tyrants to account. The right to bear arms is tantamount to the state recognizing your right and your responsibility to defend yourself, as well as your ability to hold the state accountable for continuing to recognize that right now and in the future.

Child porn is all vile, illegitimate uses. There's nothing to redeem it at all. Ever. Unless you're a pedophile. It makes me wonder about Mr Steinberg over there, in fact.