As has been hashed out on this board over and over again, getting Noel means one of two things. It means you are only getting half a season of rental of him, or you are punting on even the possibility of a max free agent next year. Even if we got a steal and signed Noel to 10m/year (probably unrealistic,) then you are saying "Nah, we will pass on even the remote possibility of trying to woo Griffin or Hayward."
Many of us may be willing to do that, but Ainge most likely isnt at this point.