Some possibilities:
1. Redshirt. We could be limiting his participation to a few key games.
2. Poor vision. You can be a tough, fast runner and still be a bad running back.
3. Poor hands. Does he drop a lot of passes out of the backfield? If so, that would make the play-callers job even more difficult. Heupel/Golesh are already limited by poor QB play. Can't afford to also coddle a RB.
4. Poor pass-protection. Same logic as above. If you aren't as good at protecting the QB as the other guy, good chance they're going to go with the other guy.
5. Ball security. We don't know how many balls he has or hasn't fumbled in practice.
6. Undisclosed injury/medical condition. The guy could have been in concussion protocol against BG & Pitt for all we know or maybe he's recovering from a pulled hammy so they don't want him going full speed. Or maybe he fell on a helmet.
In summary, the possibilities are endless.