The two top seeds have pass defenses in the bottom third of their conferences. I suspect it has to do with two things, one they both score a lot of points so teams have to pass to keep up with them. And two, the refs don't call holding and with the exception of Lions receivers they don't call offensive pass interference any more
The fact is you can't stop a great, much less good QB in this league if you can't get to the passer and it's very hard to do with the rules protecting QB's. The receivers are just too big and skilled, Calvin Rogers is bigger than most linebackers, and guys like Driver or Jordy Nelson aren't far behind. Pettigrew and Jermichael Findlay are bigger than defensive ends of decade ago. How do you cover that with a 200 pound safety?
Until the DB's catch up to the talent on the receiving end, it's going to be this way for a while. The days of defenses dominating are done, and the league wants it that way. When is the last time a big hit made Sportcenter? Probably when someone got fined for it.
The teams that win now for the most part have mediocre running games, great OL's, better receivers, and a great QB. On defense the formula seems to be get a takeaway or slow down the other team, stopping great offenses doesn't happen. It will be interesting to see if the Niners (a throwback team) can change this in the playoffs. I'm not betting on it.