Because I think they have fixed that for the most part. Take a good look at this defense when Fuller is back there and the middle is solid man. I mean they are very tough to throw on. The problem was that we didn't have 1/2 of our starting safety tandem and the dropoff is noticeable. Which is why they drafted two safeties btw... They knew they wanted to play a style that didn't suit Rapp so well.
Every time the defense has looked bad in pass coverage has been with the rook sidelined. The talent is there, the scheme is there, and Staley calls a good game with his coverages.
The real weakness is the run defense, which relates back to roster problems in the front seven. If you can run the ball that means your underneath passes that this scheme gives up will keep you on the field. This is why a Fangio defense always prioritizes stuffing the run.
But it's not gonna get better until they upgrade the linebackers. As a result come the playoffs we won't have a top 5 defense, but more of a top 10 or into the teens. There is a cap on this unit as a whole due to the quality of talent in the front seven IMO.