I don't know why(if it's psychological or physiological) but it's more difficult for a lefty to play on the right at CB than a righty playing on the left. Maybe it,s due that there are few lefties than righties so the coaches put them on the left while to find them a place they put a righty on the left and they have more time to adapt.
I asked Chiello that on twitter a few years ago when we had the debate and he told me he's need some time to adapt to play RCB. The top clubs don't have the time(weeks, months, years) to have a guy try to adapt to that role.
Also, whenever I see a lefty play RCB they usually suck ass. I remember seeing Samuel play there a couple of times and he looked like a clown.