It's still incredibly early in the season and in Napier's tenure, so that should be the default, for now.
Mullen played guys exclusively based on seniority & tenure over more talented, but younger guys on the depth chart.
As best as I can tell, the main thing Wright has on the others is that he seems to be really good at blitz protection and picking up rushers; and the fact he hasn't fumbled yet (though he was the RB on the pick 6 by Kentucky--not sure if it was the wrong route, but something was clearly off on the play). Both Johnson and Etienne have higher ypc averages though--and if that trend continues, it'll be a valid question of why Wright is continuing to be the default RB.
And I say that as someone who likes Wright