Nobody expected so many defenders to be injured, and Ancelotti certainly doesn't want to be in that situation. But strangely, when it happened, and he had the chance to fix things (when the January transfer window opens), he did not take advantage of the opportunity, and it was as if he underestimated the problems they were facing. So for that reason, Ancelotti is indeed to blame for their failure.
I think that's the reason why he stepped down, because he felt he had made a mistake that made Madrid fail to win a trophy.
Considering last season Real Madrid losses many defender players due injured and left two central defender only, in this season Madrid make huge difference way by signing two top defender for anticipating what bad happening last season leaving by many defender players on crucial matches. Madrid's management over confidence last season without signing new defender although Ancelotti have ask to Florentino Perez for signing new players at defender position.
Carlo Ancelotti step down actually not fully his mistake after Madrid failure winning any one tittle yet but he faced crucial problem with many important players have injured and make him difficult how to face most busy schedule matches. For this season, fully believing under Xabi Alonso tactician and management full trust with him after success signing three top players so far.