Advertising on national TV is EXPENSIVE. Local TV advertising is much less expensive as are outdoor and other smaller specialty channels. Any manufacturer, selling any product spends advertising dollars carefully and they market it to the audience watching the program. You won't see advertising for feminine hygiene products on Monday Night Football. But you will see Ford, Chevy, and Dodge truck commercials along with all of the beer companies.
I think it is less that the networks don't want the money as it is gun manufacturers spending advertising money wisely. Although I'm sure that is at least a part of it. In the big picture all of the firearms manufacturers combined would be a small corporation compared to GM, Ford, IBM, Apple, etc. They can't afford to spend that kind of money advertising on National TV.
And there is no need. You almost NEVER see a commercial for Jeep Wranglers. Other Jeep products yes, but the Wrangler has no competition in that segment. They sell them faster than they can make them. The same is true of firearms. They can't keep them on shelves. Advertising is geared toward people who are already planning on buying a gun. The only decision is to buy Glock or Sig, Remington or Winchester. That need is met by advertising on programs only shooters are watching.