I think its because we simply don't have the cash. Wait till next year. We'd have Ray (and possibly KG) coming off the bench cheap, a bunch of cash (thanks to Rondo's contract), an ever improving top 5 point guard, the most championships in the league, one of the most historically significant places to play for, a GM who makes smart moves (you've got to admit, the smart outweighs the dumb), a brotherhood in the best franchise.
Its understandable, given the recent shenanigans with contract talks, that they want financial security but in the end these guys play to win and until it isn't so, we're a winning franchise.