It has to be the fact he has made it well known throughout the league he wants to play in Miami next year and Miami has the space to add him.
For Boston, it's got to be a question of why waste the TPE on someone that is almost certain to play elsewhere next year.
I wonder how true this is. I know that there was a report last year that he mentioned it to other players. I can't remember who reported that. However, the only time I've seen it "reported" this year was by Kevin O'Connor, which means it's almost assuredly nonsense.
It's actually kind of comical how this rumor became something that fans think of as an absolute fact. Here's the original report:

So, allegedly, he no longer wanted to be in Indy. Why? Probably because they've got one of the cheapest owners in the NBA. However, the part about him being definitely bound for Miami is a media-created myth. He mentioned he would be up to signing in, at minimum, Toronto, New York or Miami. Who knows how many other opposing players he said "can I come play with you" to?
It's a silly rumor. Maybe he ends up in Miami. Or, maybe he just wants to play in a good environment where he can win.