Because he's a B.A.M.F., that's why.
Well, that combined with his track record. Once you mixed in his two crushing knockouts over Terry Martin and Matt Lindland under the Affliction banner, it was enough to get him back into the world's largest fight promotion, where he had formally ruled the roost at both heavyweight and light heavyweight.
A special set of circumstances, largely the fact that Anderson Silva had all but completely run out of challengers to his 185-pound strap, made "The Phenom" a hot commodity, as the UFC went full press with a big marketing push to reestablish his name with the newer Zuffa zombies and remind old fans of the fear the name Vitor used to strike within the world of MMA.
Of course, it would have gone over like a lead balloon if he had been given an immediate title shot, so an opponent was needed to test his might against a top level fighter that would provide him the proper credentials to challenge a champion the caliber of Silva.
Paging Rich Franklin.
Of course, another special set of circumstances, like the fact that "Ace" was on his way up to light heavyweight and made a pit stop at 195-pounds as a favor to UFC, meant Belfort would still earn his shot at "The Spider" without winning a middleweight fight inside the Octagon but why argue semantics, right?
As it turned out, Belfort came through in spades, feeling out Franklin for the first few minutes before exploding with his trademark lightning fast hands and scoring his third consecutive knockout to send him on his way to a date with Silva for the middleweight crown.
Check it out after the jump ... just don't ask about the eventual result of Belfort's fight against Silva.