Rosters are terrible. Managements/0wners/Coaches are terrible. Neither are remotely close to winning and their future prospects are not good. Also some of the perceived advantages regarding marketing have been lessened in the age of internet and cable.
Celtics are in a different situation, but given a choice between a team ready to contend and one that's not, a FA who is getting maxed-out wherever he goes is going for the win now proposition. We would be in much better shape, however, if the cap rise coming from the new TV deal was smoothed as the NBA wanted, but which the players union chief rejected.