Schumacher seems to have lost the ability to perform right when he needs to - something Vettel and Rosberg are insanely good at. Neither of them do particularly great laps in Practice, Q1 and Q2, but in Q3, when they really need to, the performance is suddenly there.
In addition, Schumacher almost always manages to muck up his laps in Q1 and Q2 which then forces him to do another run, which then hurts him even more later in Q3.
Sure, half of the time he's got some kind of technical problems that are not his fault (DRS not working, KERS not working, vibrations in the tyre etc.) but Rosberg has problems too sometimes and apart from Japan, where he couldn't drive at all, he still managed to put the car into Q3 every single time.
Still, if Mercedes had the fastest car, I think Schumacher would beat Rosberg to the WDC.