It's harder to blast the NRA when you're actually talking about millions of law-abiding, rights-exercising citizens.
The big, bad, evil gun companies are a much easier target to lambast. Never mind the fact that many of those boogieman companies turn a profit by arming those same millions of citizens.
It's just another avenue for hoplophobes to attack something they don't like.
Roughly 85% of the NRA's money comes from sources other than gun manufacturers.