The only thing I know about submarines is from watching "Operation Petticoat." That said, here are my suspicions.
The US has designs that provide more protection than a double hull, such as internal compartmentalization. The double hull significantly increases size and weight. Imagine driving a car with a second larger body and an air space in between the bodies.
The double hull significantly increases size and weight.Larger size and weight is not a major concern for the Russian sub builders, because they believe a larger sub is more survivable.
For example, the Oscar II class SSGN (nuclear cruise missile submarine). Its heavy displacement and double hull enables her to take three direct torpedo hits to sink.
The outer hull acts as an armored shell, much like a turtle shell for a turtle.