$$$
Nothing else.
Anderson Silva was handed a rematch because it was a good business move. Plain and simple. No fighter who's coming off a convincing loss, and let alone a knockout loss deserves a rematch. Fanboys will claim he was given the rematch because he was a dominant champion, Dana will say the same, but if we review history you won't find many (if any) dominant champions that were given an instant rematch after a fair loss.
Truth is businessmen could care less about who deserves what, title shots are often given based on marketability, and there's no bigger fight at MW that a rematch between Chris and Anderson.
Let's consider the possibility that Anderson might be on his way down and about to have a streak of losses. Strictly from a business perspective, do you think it's a smart choice to have the guy you hyped as #1 P4P and GOAT losing to some other contender? or is it better to immediatly match him against the champion and hype it as the biggest rematch ever? The answer is pretty obvious.
The UFC needs to make as much cash out of Silva while they can, thus an immediate and undeserving rematch was handed to him.