We didn't get the basics right defensively.
Kompany has been our only world class defender and he's been crocked. Without him, we don't have a single world class player in the back four - MDM, Zaba (crocked for most of the season) and Clichy were once world class but are getting long in the tooth, Sagna has been impressive, Otamendi can be good but is inconsistent and Mangala and Kolarov aren't good enough. Those ingredients don't add up to a solid defensive unit. The problem has been exacerbated by Pellegrini's insistence of playing a midfield two too often and giving the back four too little protection.
If we had had a really solid back four all season we would be challenging for the top spot now at the very least.
Pellegrini and Txiki take the blame, of course, but I think some of this would have been hard to predict. Last summer, Kompany was working hard in pre-season, Zaba an MDM were fit and impressive, Mangala had had a strong end to last season...