The knee-jerk response to this is typically "Because Vince," and while Vince's heroics were a critical piece of it, there's more.
warning: detailed longcat commences; grab yer adderall and try to stay awake
The still oversimplified but much longer story is that he got lucky with some changes to his staff. (To be fair, he had some hires of his own that were good. Akina was a great hire; Tolleson was fantastic his entire tenure here. And... well. That's the lot.)
Hiring Dick Tomey. I've been beating this drum ever since it happened, and honestly it basically happened on accident. Tomey changed the way Mack had done things for decades, including having starters go against starters in practice. And revamping his defensive staff in general was needed, because his initial hire was a gadget coach who made his name by stopping Spurrier's Florida teams. Mack had called Tomey up to see who he should hire to coach DE's, and Tomey told him "Hire me." He, SB-winning coach Greg Robinson, and (already on staff) Duane Akina then helped build some championship-winning defenses. Tomey was the only person who could tell Mack to do things differently.
McWhorter coaching Nunez's recruits. When Mack started, he hired Tim Nunez as OL coach, who was a top-notch talent identifier/acquirer who couldn't develop that talent once they arrived; McWhorter could develop that talent once on the team, but was lazy and awful at identifying and acquiring said talent. The 2005 season featured a starting OL with Nunez's recruits and McWhorter's development. They were awesome and it was never as good before or since.
These things were more or less Mack catching lightning in a bottle, things that broke him out of his normal tendencies and finally got him that gold ring. But he himself didn't recognize how critical they were, and after Tomey left, he eventually went back to the ways he had done things before, as if to prove that he could do it his own way. And that might have worked in 2008 and 2009 if one thing had been different: If the OL recruiting had been as good under McWhorter as it had been under Nunez (again, with McWhorter's development). Without a good enough OL, poor Colt McCoy was destroyed in the Big XII title game (where we barely eked out a win) and completely ruined in Pasadena (which by itself probably led to the loss).
I mean... the fact that Mack thinks he should've given Garrett Gilbert more snaps, instead of, you know, maybe getting some badass bruisers on OL in the preceding years over Good Kids Who Graduate, just shows that he still has no idea how he did it in '05.