This Pittsburgh team does not run the ball for TDs with Big Ben at the helm. His ego is too much.
Willie Parker
2007
Pittsburgh Steelers 15 321/1,316/2
2005
Pittsburgh Steelers 15 255/1,202/4
Mendenhall only had double digit TDs once, same for Willie Parker. That is three every down backs over the course of 10 years (Parker, Mendenhall and Bell) with just two double digit TD seasons.
Double-digit touchdown seasons are quite uncommon events. Since Roethlisberger took over in 2004, there have been 88 double-digit rushing TD seasons, or 2.75 per team. The Steelers have had 3, which is just above average. The Steelers also had a pair of near-misses- a 9TD season from Bettis and another one from Mendenhall. If you include those near-misses, there have been 3.7 9+ touchdown seasons per team and Pittsburgh has 5.
Since Roethlisberger took over, Pittsburgh's RBs have rushed for 117 total touchdowns, which ranks 14th in the NFL right between run-heavy Ravens and run-heavy Broncos. I doubt many people would suggest that the Denver team or the Baltimore team do not run the ball for TDs. Green Bay ranks 24th in rushing TDs over that span while New Orleans ranks 7th, but I doubt anyone would use either point of data to make sweeping generalizations about Aaron Rodgers' or Drew Brees' egos.
In fact, since 2004 the Pittsburgh Steelers actually rank 5th in the entire NFL in number of runs called inside the red zone, (versus 10th in passes called in the red zone), which hardly seems like the sort of thing you'd expect if Roethlisberger's ego was not conducive to sharing the glory. In fact, it more paints a picture of an RB corps that had plenty of opportunities but failed to make the most of them; that it's the RBs' own failings, and not Roethlisberger's, that led to their low TD totals.
The Steelers are hardly the Vermeil/Holmes Chiefs, the Alexander/Jones/Hutchinson Seahawks, or the Tomlinson Chargers, and Bell seems an unlikely candidate to lead the league in rushing touchdowns, but there's no reason at all he can't experience a normal amount of success given his success everywhere else on the field. Even if, over a tiny 6-game sample size, his touchdown totals have been underwhelming.