The first, and most obvious, is the Colts' considerable lack of depth at running back. Trent Richardson and Dan Herron are joined only by undrafted rookie Zurlon Tipton from Central Michigan, currently the third running back. The Colts, like most teams in today's NFL, prefer to play a rotation of running backs. With Ahmad Bradshaw on injured reserve, there aren't a lot of options.
Arguing for more carries for Herron is one thing – he's been more productive – but to suggest that he alone handle all the carries, when he has no history of being an every-down running back, is a stretch.
But there's a much bigger reason behind the Colts' rationale to stick with Richardson. And it has little to do with running the football. Richardson is the best pass-blocking running back the Colts have, a distinction that greatly increases his worth to the coaching staff.
You might not value this quality in your favorite team's running back, but quarterbacks do not share your opinion.
While it's just one opinion, it makes a lot of sense. Protecting one of the top QBs in the league is key to the Colts success and playoff future, and if Richardson has the best ability to keep Luck upright, he's going to be on the field. That could mean on the field in key third down situations, which obviously would limit Herron's pass-catching numbers. Both Herron and Richardson had two receptions last week.