Everything is so cut and dry in hindsight. Danny Ainge is in charge of seeing the big picture, not just the first round of the playoffs. Both Birdman and Kenyon have well documented issues off the court, Danny has to weigh the benefit of what they could bring to the court versus issues they could introduce to the locker room. We've had our fair share of knuckleheads over the past few years, Danny and Doc know how much goes into managing delicate personalities.
Here's the cold hearted truth: we are not a Chris Andersen or Kenyon Martin away from a championship. We are not legitimate contenders. When it came down to it, perhaps Danny felt that signing either of those guys wasn't worth the possible disruption in the locker room and influence on young guys like Bradley, Melo, and Crawford.