They've been helped by playing teams that aren't good against the run.http://www.footballoutsiders.com/stats/teamdef
Baltimore is 25st. Cincy is 29th. NYG is 22nd. Cleveland is 14th and the Steelers couldn't run the ball even when they weren't fumbling. They had trouble running against a bad KC rush defense. Maybe it hasn't been a weakness but it hasn't been anything noteworthy. The offense will go as far as BRoeth can take it while avoiding the mayhem caused by Colon.
Fwiw, official NFL stats show team run defense as CIN #11, CLE #16, NYG is #21, BAL #23. Not hugely different from your numbers, except for the Bengals. Officially, Denver and Washington are both top 10 run defenses. In week 1, the Steelers only rushed for 75 yds against the Broncos. But in week 8 they ran for 140 (including over 100 from Dwyer) against the Skins.
I do agree that Roethlisberger is the key link in determining the Steelers' fortunes..... however, with no idea of what his effectiveness will be when he returns (I'm expecting him to be no more than about 80% optimal for pretty much the rest of the 2012 season, however long that is), I think the biggest boost he can have is an effective run game. And I think that week 8 performance against the Skins is the telling one for where this team's ground game can go, and I would say probably where it has to be for the Steelers to have a chance in the post-season.
As with anything, it's a mutually beneficial relationship. Ben makes teams respect the passing game more, which opens up opportunities for running against favorable defensive packages. You saw with Leftwich and Batch that teams didn't really respect the pass (and why should they?) and routinely loaded the box, so the running game was heading into 7-man fronts a lot.