Because he has no competition whatsoever. I wouldn't say Ogbonna is much behind him at all, especially if we're talking about 4 man defense.
So? Just because it happens once in a blue moon doesn't mean we should keep him for 4 more years when he might start playing better. The guy doesn't have much of a brain, there's no way he'd improve. He has been here for 4 years. He did improve but he still has brain farts and by far the most mistakes in our whole team.
But of course we're not gonna sell him. There's a simple reason and it's that there are no teams who'd buy him, not for that big cash we bought him, let alone more.