And Overeem hadn't? Fedor was coming off his first legitimate loss in 10 years, which most people were, while not taking anything away from Werdum, considering more of a mistake on Fedor's part (not keeping the fight standing) rather than the unstoppable grappling of Fabricio Werdum (since, you know, Fedor's dealt with high-level grapplers before). A single loss like that people can look past, a minor hiccup, something that was mathematically inevitable considering how long he's fought. Wouldn't effect his standing as the world's best heavyweight.
Bigfoot then went and manhandled Fedor in a way never seen before, with Fedor getting beaten and bloodied up more than he ever had. And this was by Bigfoot, while a decent fighter, never seemed to have the respect or "pedigree" of Werdum.
That was the beginning on the end. Not that Fedor lost, that Fedor lost twice in a row, and how he lost.