not every coach(literally none) tries something new each season when they already have a winning formula.
and as much as I (and many others judging by their comments) liked to see a change to 4-3-3 in second half, there were always downsides of limiting tevez's free role, players taking time to adapt in little time and like Dady-SK said not having the right players available (Pepe)..
having said that, we used to be prepared for such occasions, by the looks of it he's spending too much time perfecting the 3-5-2 in training and the results haven't been as rewarding.