Struggling to understand why Rex is our #1 option

The guy is old, he's slow, and he's clearly washed. Pierce is young, strong, and has showed a lot of promise. Why is the coaching staff still insistent on Burkhead being our primary runner? He wasn't even our main back last year, and was limited to gadget-type plays, which is how he's been used for pretty much all of his career. If the goal is to give Pierce limited touches and ease him into the role, then why did we not stick with Marlon Mack? He wasn't great in the preseason, but he still looked a lot better than Burkhead. I'm just really confused with this personnel decision