Pittsburgh has a top 5 OL IMO, and probably the best 1-2 WR group in the NFL. And also a top 10 QB. Leveon Bell is basically a slightly more gifted Justin Forsett, he's a smart runner and a great fit for their system, but he's not an elite talent like Lynch, AP, or Murray.
I'm not surprised at all that other RBs have had huge success in that offense. It's the best balanced attack offense in the NFL, and the OL they have is under-rated. I am a little surprised that Williams is having such a great year though. He looks like he's lost a half-step, but he still dominates because the system is so awesome.