The man, frankly, wore out his welcome with the Southampton away match eradicating any hope most Blues would ever support him again. It was a disgrace for a professional to be insubordinate like that. That's really it. There was a bit of a debate about left back early in the season when Clichy was hurt, and he started off well with Sterling, but then his issues came to show themselves, and once Clichy was fit enough and got a few games, became first choice even though his crossing is no where near the same level.
Sure, Kolarov has made some key contributions with his wicked left foot that helped us win trophies. Yet, I almost always preferred Clichy, who is not always so consistent, but Kola, as a defender often left us exposed with his low work rate and lack of pace. Under Pellegrini's system of overlapping fullbacks it could work when we dominated possession with opponents on the back foot, but hurt us when we couldn't or were hit on the counter with pace, where his weaknesses could be exploited, compared to say Zaba, who pushed forward, but did not hurt us as much over during his prime years due to his commitment to defending.
Perhaps offering Kola a new contract was not so wise when we did, and for the amount, but FFP I think was limiting our options in the transfer market, so we went for it. We will probably have to take a hit either paying part of his wages or at a discounted fee.