Why would we want those three guys to get beat up early in each game by fresh first team starters in the regular season? They would all end up on IR after the first 4-5 games.
Would any of them be a better choice against a top team on 3rd and 1? These RB's are more specialized and none of them are the physical choice to run inside against the monster DT's that most good teams have.
Zeke hasn't done anything special because he doesn't have anything special to prove in the preseason. He's not close to being the dominating back he once was. Considering the wear and tear of 8 seasons, 120 games and over 2000 carries, the wise move is to save him for the appropriate plays in the games that matter.