Such an ad would be polarizing. Many people to gather and watch the Super Bowl; sometimes with friends and friends of friends, sometimes with extended family, and sometimes with total strangers at a bar with a big screen.
If CBS ran this ad it might be counter productive as conservative viewers shouted BS in unison.
Also if CBS let this stand then they would be faced with the prospect of airing political ads during similar programming. It would make it difficult for them to refuse to air NRA, pro-life, etc. ads without showing their bias.
Well stated, weegee! Let me add to that that CBS , like all businesses, looks for repeat customers. Coca Cola and Anheuser Busch are ones. The MoveOn hippies? Foggetaboutit!