I might take some flack, but whatever. A big problem is Russell Wilson.
A lot of the issues in his game that people have pointed out: Timing throws, throwing from the pocket, conservative throwing into tight windows. Those issues become magnified inside the red zone where the field and throwing windows become even smaller.
All the open space between the 20s that's available for Russell to run when his receiver carry their defenders deep? That's gone, as well.
You rarely (if ever) see him drop back and fire it to his receiver with the timing and precision that a lot of other QBs have. Personally, I think his size has hurt him here. I've played QB and am the same height as RW. When you get closer to the endzone it becomes more clustered. It's just physically tougher to see things over the middle. That's why you'll see a lot more corner routes and out routes than you see slants. Or plays schemed to get players open (rub route). Or more TD's passes when Russ scrambles as opposed to from the pocket.
Add in an offensive line that struggles to protect, has struggled more than usual run blocking, and commits at least one stupid penalty in the redzone a game, thus killing your chances for anything more than a TD; you got a recipe for disaster.