That's what Ainge should have asked for. The Noel + grab bag offer took up $3.5 million more in cap room and at least one extra roster spot. Ainge values three things this summer:
1) Top line talent. Brown beats Noel on that.
2) Cap space. Currently we have about $800k extra room after signing two max players, which is clearly what Ainge is shooting for. (Assuming Amir and Jerebko are gone). Given that the Philly offer took up that much extra cap space, it would have necessitated further trades to create the room. On the other hand, Noel + Lakers pick only takes up $400k more cap space, and so does not create a problem.
3) Roster space. Even after three draft and stash selections, and two picks traded away, we still have a very full roster. It wouldn't surprise me at all to see someone with a guaranteed contract get cut in training camp. A 1-for-1 trade would have been fine. Ainge would prefer a 2-for-1 I have to imagine. But a 1-for-2? That's the wrong trade.
If the Sixers didn't want to part with the Lakers pick because of the chance it's a great pick next year, that's their prerogative. But it's not Ainge being stubborn. It's Ainge evaluating what he thinks best helps the Celtics both this year and in the future. He is under no obligation to make a trade -- no team is. The Sixers offered the best they could that fit in with their team's needs. That offer didn't match up with the Celtics' needs. And so no deal was made. And that's how most trades go, or rather, don't go.