The parcours was overly favourable to Wiggins. No HC mountain finishes, 3 mountain finishes, only 2 of which where real mountain stages, still on powerclimbs. Outragious amount of tt kilometers compared to that. The best GT contenders in the world at that time weren't there or were underperforming. Contador and Andy Schleck, the two strongest Tour climbers at the time, weren't there. Evans, defending champ, wasn't too good. His strongest competitor was Froome, and he was his teammate. And as Wiggins was the VIP in Sky, he had to win, and Froome was on the leash. These are all factors decided before the race. competitors had to make up minutes in the mountains without opportunity to do so, not to mention Sky's ridiculous performance.
Sastre actually had to beat other riders. All the great GT contenders were there, except for Contador and Astana. Sastre actually beat someone, (Evans, who had come 2nd before and would later win a tour) Wiggins was attacked once, and that attacked was stopped by a radio call cause you know, it was his own teammate. Nibali never seriously tried to beat him, he was quite happen to podium. Froome wasn't allowed to try and win. JVDB (lol, he never actually tried to win in the Tour) was more than happy to finish 4th, as was TJVG to finish 5th.
Not to mention Wiggins later attempt to win the Giro, which he was sure he could do and which he epically failed in every possible way.