There are a handful of elite RBs, many of which were not projected to be elite coming out of school.
Then there are a load of good backs. The difference between those backs is who does the little things best, pass blocking, taking care of the football, catching out of the backfield, that kind of thing.
Then you factor in the fact that even for the really good backs, the shelf life is not that long. That's why you see backs go from nothing to something almost overnight and then quickly back to nothing. Happens all the time.
Dixon may just need the right break, or he may never really shake out in the NFL. It's hard to say. I figured Green-Ellis wouldn't last all that long, but clearly he does the little things well, and he runs hard and does a good job getting what's available. You can make a career doing that.