I loved the SS-21s. That being said, they were for a different world. They were designed to go after Soviet fleets and gut them while having the ability to also go under the ice cap and find Soviet boomers.
The USSR collapsed and gutted their fleet without any help. The Russians also followed the late Soviet strategy of keeping their boats in easily defended bastions rather than pursuing the under ice strategy (probably a good idea since the ice is getting a bit thin).
With both reasons for existing gone, it was almost impossible to justify the cost of the SSN-21 boats. Procurement of 12 hulls was projected to price out at $33B (in 2002 dollars, right around $40B in 2013 dollars). The 774 Class (Virgina class) runs around $2.7B in current dollars or 19% less than the Seawolf and her sisters, despite have much more advanced electronics and sensors. Put at the most basic you can have five Virginias for the cost of four Seawolves. When you need 60+ boats, that is a serious savings.
The 774s are reputed to be more maneuverable, and even quieter than the SSN-21s, while being noticeably faster and having a greater test depth (good luck on finding out either figure, the USN hasn't changed the "official" published numbers since the Skipjack (SSN 585) and her sisters were launched).