Rondo is a guy who wants to and obviously thrives in an up tempo, early offensive system. Ray needs time to clear screens and get open,which he doesn't do nearly as well as he did a few years ago.
Now who should you build your offense around? A mid 20's All-Star PG who is just entering his prime or a 37 year old shooting guard who is past his prime?
The team as a whole looked a whole lot better playing Rondo's style with Bradley then it did walking the ball up the court waiting on Ray Allen.
On top of that Ray wanted 3 years at 9 million per year so he obviously didn't want to be in Boston any longer regardless of what he said publicly.