There are multiple reasons, but to boil it down the #1 factor is the Offense is on a tight leash, and is not allowed off of it until it gets down in games, or is in a 2 min situation. Once it gains the lead it goes back to being on the leash. This leads to a ripple effect that causes or magnifies the other problems on the team.
Beast Mode in Super Bowl 48 sarcastically, but truthfully asking Pete Carroll if it is OK if we can score more points? Pretty much summarizes it.
This philosophy can work if you have a GOAT Defense, but if it is not GOAT all it will do is cost you games, and lead to mediocrity. Which is what Pete Carroll was in the NFL before Russell Wilson came along. Why Russell Wilson? Because he has been incredibly efficient and makes so many explosive plays given only 25 throws a game. It allowed / allows Pete Carroll to have his cake and eat it too, but if the D doesn't return to form in a hurry his philosophy will crumble, and lead to 7-9 to 10-6.
I've seen some comments on .NET about the Defense blowing 4th quarter leads, but it is cause and effect. Because once the offense gets the lead in the 4th quarter it goes back on the leash.
#2 Darrell Bevell. Bevell has proven on many occasions he doesn't know how to game plan, and attack opponents weaknesses, and avoid there strengths. Instead he does the opposite thinking it will surprise them. His Red Zone / 3rd down play calling and scheme is some of the worst stuff you will see in the league. Beast Mode, Jimmy Graham, & RW on the same team, should not be as hard as the Hawks make it out to be on Red Zone / 3rd down situations.
#3 The Lack of a strong leader on defense (Kam Chancellor). I see guys that are worn down by having to bail out the Offense week after week (leads back to #1 & #2). While also being fat & paid. (The highest paid D in the league.) They need a guy to kick them in the butt, and scream in there face, do Pete Carroll's dirty work, but it can only come from a guy like Kam. E.T. is in his own world. Sherm is a corner. Bobby Wagner is a quiet guy. Mebane is quiet also. Michael Bennett is crazy, and funny (The class clown.) Kam is the only guy personality wise that has leadership qualities on defense. This can be overcome if you play more aggressive on offense and score more points, but if PC refuses to do so = mediocrity.
#4 The Offensive Line - From all aspects: the available talent pool, scouting & drafting from that talent pool, development, free agency, & coaching. This is a league wide problem, not just a Seahawks problem.
#5 The schedule: You couldn't have drawn up anything worse than Rams on the road wk1 10 AM with a green offensive line, and the Pack week 2 on the road. Overall it is one of the tougher schedules in the league, but the worst is behind them. There is no need to panic yet. The Seahawks can't afford a home loss at this point though. The Hawks have to play 8-0 home ball, and find a way to go 4-2 on the road the rest of the way to get to 12-4. That should be good enough for the #2 seed, and another crack at the Pack in the NFC championship game.
#6 Pre-snap Penalties - the Okung false starts, and the Michael Bennett off-sides is getting tiresome, the rest of the penalties I can live with because they won't call it every play. You can actually gain an advantage.
#7 The loss of Dan Quinn. I saw the return of the Gus Bradley soft zone 4 qtr D in this game. What I loved about Quinn is he would go man, or press bail 90% of the time, soft zone generally only appeared as a change up, or if they had a big lead. Dan Quinn's D-Line background was a perfect complement to Pete Carroll's defensive back background as well.
#8 The loss of Ken Norton. Ken Norton filled the vocal leadership void of a MLB. Also just look at the guys Ken Norton has coached. He just knows how to turn seemingly anyone into a stud linebacker.
#9 Russell Wilson - suffers from mobile QB syndrome. Which is where a QB with mobility is hell bent on proving he can win with his arm and not his legs, thus surrendering the better option on multiple plays. Aaron Rodgers meanwhile had no shame tonight he was looking to extend and scramble on nearly every given drop back tonight. Part of the game plan appeared to get Russell Wilson's legs involved early. Wilson didn't want any part of it. He waited until the 3rd quarter got the lead then the offense went back in the tank (see #1 & #2).
#10 Marshawn Lynch - He has lost a step. He hasn't fallen off a cliff, but he has lost a step.