Wins and losses: The Cardinals went 5-2 with Skelton starting last season. They went 3-6 with Kolb, with one of those victories coming when Skelton was the primary quarterback. While Skelton benefited from defensive improvements late in the season, he also benefited from the bottom line. The Cardinals won more frequently when he played. Kolb didn't do enough this offseason to alter the perception that the team responds better to Skelton.
Durability: Skelton is 6-foot-6 and 244 pounds. He's sturdy and has proven durable to this point. Kolb (6-3, 218) has had a hard time staying on the field. Arizona's ability to protect its quarterbacks was in question even before starting left tackle Levi Brown suffered a torn triceps tendon. Skelton has a better chance at holding up.
Pressure: The contact Kolb signed last offseason put pressure on him to play at a high level. Kolb admittedly pressed. He said he was working this offseason to achieve a freer, let-it-rip mentality. That pursuit proved fruitless from the beginning. Kolb did not appear comfortable when under siege against New Orleans in the exhibition opener. The injury he suffered early in that game didn't help. Arizona figures to need both quarterbacks this season. Perhaps Kolb will fare better coming off the bench, without so much pressure.
Going with Kolb would have felt like an organizational decision based on how much the team has invested in him. It's difficult for an organization to bench a player it once planned to build around. However, going with Skelton feels like the right move on the merits.