Because it's difficult to accomplish that in a 30 to 60 second ad. The talking heads at MSNBC droned on for several days about how ugly and mean spirited was the NRA ad. I take that as a sign the ad worked, because MSNBC had to work so hard to discredit it. Just as some here on thefiringline are doing. I loved the NRA ad, because it is important to demonstrate the hypocrisy of the anti-gun movement; and that the NRA understands, we are in a dog fight.
First off, no one who has posted so far are trying "discredit" the video, or the NRA. The NRA is not the end all, be all of gun ownership, and blindly cheering for them no matter what they do is what sheep do, not reasoning adults.
The NRA could have made any video, with any message they wanted, that supported the RKBA. They chose to make a clumsy election-style attack ad, using one of the weakest arguments they possibly could have made. And they released it BEFORE the President announced his EO's, and before he announced his recommendations for new legislation. They were made into fools, because one of the President's EO's involves providing "incentives" for hiring resource officers (armed LEO's), which is what the NRA claimed Obama didn't approve of. He probably tacked that one on just to accomplish this tactical victory.