There is something much larger at work here, at least in regards to the Bengals...
The browns, the chiefs, the bills, etc... never had a "lost decade" OK, so the browns are getting there. The Bengals were not only the worst franchise in football for most of the 90s thru early 2000s, they were one of the worst run franchise in all of sports. It was a competition b/w them and the Clippers over who was run worse during that time period. The Bengals lost an entire generation of young kids growing up during that time period, fans that would currently be the core group of season ticket holders.
When Marvin came to town, the drafting got better, the on field results started to get better, but then they regressed hard a few years back, and there was never any success in the playoffs, never any lasting feeling that they were better than the Ravens or Steelers for the foreseeable future and worthy of investing in a pair of season tickets. Just when the Bengals started to win back a new generation of fans while re-capturing some of the old ones, the success was ripped away, not to mention the poor publicity of having guys run afoul of the law, Palmer's public trade deamands, the general jack-assedness of TO & Ocho, etc...
They started to get them back last season, but they're very much in a fool me once, shame on you, fool me twice, shame on me mode. There was some excitement at the start of the season that was quickly sapped by the destruction of the Ravens on opening night, and later the losses to the Dolphins & Browns. If they want to win the fans back, they'll need more than a 2 game winning streak.
In Kansas City, you have generations of fans who grew up going to KC games, because that is what you did on Sundays. You tailgatted w/ the same family's/friends, and your kids carried on the tradition. The Bengals lost that during the 90s. The cost of the NFL is so expensive, that if your relying on single game ticket sales, you're going to struggle. this team may well be in the playoff hunt, but its almost by default of being .500 vs a significant # of wins vs top competition. Other than the win over the giants, they've got a win at home vs a poor Browns team w/o their top corner, a win over a rookie QB in Washington, a win at a bad jacksonville team and a win at a bad KC team.
I want to believe, I want to hope, but they've gone little to prove to me that they're poised for a repeat trip to the playoffs. And that is why they're not selling out vs Oakland.