He's cost teams more points in fuck-ups than he's gained them with saves the last few seasons.
He WAS a decent keeper. NOW he's a liability.
This is it for me. Paul Robinson, Rob Green - similar circumstances. Keeping goal for Man City with a more than decent defence in front of you (even allowing for the loose cannon of Jo Lescott) is a big insurance policy. He clearly had proved himself prior but far too many howlers (and there were many) at international level sealed his fate. Peps axing was borderline ruthless/positive but it was always in his own capability to prove himself again.
To be fair, the move to Burnley could have gone in his favour - hardly the Prems most watertight defence so plenty of opportunity to show your worth and people do know the difference between a keeper exposed far too often and having no chance to one that is capable as a last line.
It goes without saying, confidence plays a huge part in any sprotsman/woman's armour and that can be an enormous battle to regain if there becomes a crisis with that confidence.
To sum up - He clearly had 'it' but once 'it' was lost he struggled to find it again.