I think it's a mixture of a few things. George is a better fit for Stevens offense as you said. He's younger, taller, better wingspan. There are rumors that he's not the best locker room guy.I remain a little big puzzled why the Celtics do not appear to have Butler ahead of at least George (Hayward comes 'free' so I get that). But, it seems they prefer George---that may well be about spacing and fit with Stevens' offense.
However, I think the most important thing is that Butler is a ball stopper and very ball dominant. I don't think Butler fits very well with IT.