To answer the question of the OP. It's a combination of things. The Colts offensive line is built to pass block and not run block, and they don't even do that successfully. the amount of runs in the play book are limited, and he shares them 50/50 with Ahmad Bradshaw. The run designs are repetitive and unimaginative. Plus the Colts don't want to scheme for a player that will take anything away from the golden boy in Andrew Luck.
I love my colts, and almost as much as I love Bama. However, I am infuriated by how poorly the coaching staff uses Trent, and I honestly wish we would just let him go somewhere where he can be successful. Trading a first round pick for him was an atrocious move because we don't use a RB enough to justify losing a 1st Rd pick. I loved it at the time, but the farther we get from it the more I realize how terrible of a move it was.