In the forums, I've suggested that Ainge decided to tank after Rajon Rondo was deemed out for the season. I want to flesh out that theory here.
In late March, Jackie MacMullan reported that Kenyon Martin said he was close to a deal to join the Celtics and that Doc and the players wanted him. He gave the timeline as "a couple of months ago". What happened a couple of months before that statement? Rondo went down with an injury.
It is reasonable to speculate that when Rondo went down, or certainly by the time Jared Sullinger was lost for the season six days later, Ainge went from believing the team had at least a longshot chance to believing it was at best a 7th or 8th seed and the team was better off slipping into the lottery with a slim chance of the ping pong balls allowing him to reload rather than rebuild.
That shift in Ainge's assessment would be a reasonable explanation for why the team didn't pull the trigger on brining in Martin, who would only be useful if you were planning a playoff push. It would also explain why Ainge may have been dragging his heels on filling out the roster, with the team frustrating his goals by winning too much despite injuries.
If Doc wanted Martin, one wonders how often he has clashed with Danny on personnel decisions and whether that motivated Doc getting control over personnel decisions if he went to the Clippers, according to reports.