He was supposed to get goal-line carries this year but the Steelers have either got a ton of bigger passes for TDs or play-action at the goal line. Davenport isn't stealing TDs; Parker just isn't getting in.
It's been bad luck more than anything that is keeping Parker out. He still is getting a ton of carries and has gone over 90 rushing yards in 7 out of his 11 games this season.
One thing I have noticed is that Parker struggles against bad run-stopping teams for whatever reason. I'm guessing it has more to do with other teams stacking the line and forcing the Steelers to throw, which the opposition thinks is the best way to counter the strong running game of Pitt.
His best fantasy game of the year was against the Bengals who are coming up this weekend. Apparently the field may be crappy again but I'm not convinced even that can help the Bengals do anything on defense.
You may be let down again but you almost have to start him this weekend. He does have some tougher matchups coming up though and if you can trade him still and get decent value, it may be worth it.