Why isnt reggie bush the starting running back for the saint?

Bush doesn't have the endurance to be an every down back. He would have the skills and power to make an impact almost every down, but his stamina won't allow him too, he makes a pretty good 3rd down back for short yardage though and that is his role on the Saints.. plus the Saints have a pretty decent HB already... if Bush was their only option they would probably stretch him more, but since they don't have too they can utilize him in a role that suits him better. I can see them actually going 60/40 or even 50/50 split as some programs like the ravens and cowboys do, but that is still pushing Bush's limits.