If we still assume that both are just puppets of a higher level power, what I imagine it is just that in order to implement new rules, changes and restrictions, keep everyone under control, without the people revolting "too much", the way to do it is putting a new face in power, with a softer and calmer image than Sarko, so the masses will not fire at him as much as they would do if Sarko would have "won".
My personal opinion is that Hollande has been prepared to be the next president since years ago. When Royal lost and they broke up, it was just too "good" that a new partner, with the right skills, knowledge and contacts came into his life (his current girlfriend, an "agent" inserted in his life?). She made everything to put him into the race, althought they were always behind DSK.
They admitted that a new look was needed in order to create a better candidate, and even a new "personality" (that was said in the news I promise, call me "suspicious" :-)) had to be created.
He has worked behind the scenes, because DSM was supposed to be the next president defeating Sarko. Then, oh surprise, DSK is charged with lots of stuff and he has the route clear to be the candidate of the PS.
I'll probably be wrong, and he may be a good souled person trying to fix a difficult situation, but all that crossed my mind yesterday, the more info about Hollande was being said in the news, the more I was conecting ideas in that sense. Guess I do not trust anyone in power to be a "good person" anymore!