I think it's because it just went a bit too far back in time (literally, to the 50's) to the point where R & B was almost indistinguishable from rock & roll. If you think of something brilliant like Jimmy Robbins - I Can't Please You, which people call R & B but I thunk is out and out Northern, then compare it to say Charles Sheffield, one is NS, the other is R&R.
I like it but I think maybe some have pushed it to it's limits and it could live a happy life away from NS, and probably does anyway.