He was a 6th round pick for a reason and he is not the type of RB that Shanahan was looking for in the read-option scheme.
If he wasn't the type of RB Shanahan was looking for, Shanahan wouldn't have drafted him. Shanahan is a big fan of getting players who fit his system late in the draft, sort of like how Pittsburgh used to snap up all the 'tweeners late in the draft in the late '90s to early '00s when they were one of the only teams running the 3-4 (Carlos Emmons in the 7th, Jason Gildon at the end of the 3rd, Joey Porter in the 3rd, James Farrior on the cheap after the Jets let him walk, Larry Foote in the 4th, Clark Haggins in the 5th, James Harrison as an UFA). Getting drafted late didn't mean that Pittsburgh didn't value them, it meant the demand for them wasn't as high because their skills didn't fit in nearly as well with the rest of the league. Similarly, Shanahan has always believed that he could take slow-but-decisive runners with good vision and turn them into relentless chain movers. The rest of the league didn't really value them because of the lack of speed or "wiggle", which means they were available cheap... but they fit splendidly with what Shanahan was hoping to accomplish with his running game, which was basically just moving the chains, leaving manageable conversions, and keeping the passing game ahead of schedule.Alfred Morris fits what Shanahan wants out of an RB to a T. Not a lot of home runs, just a steady stream of hope-killing, chain-moving singles and doubles, keeping his QB out of obvious passing situations (and therefore freeing up the QB to pick his spots to go yard without the defense teeing up on him). Edit: the fit is doubly appropriate, given that Griffin throws perhaps the most accurate deep ball in the NFL. If you give him a lot of 2nd and 5s or 3rd and 2s to play with, defenses are going to get burned, especially given how much respect they have to pay to his legs.