As fans, Noel looks appealing, but the C's almost certainly have a lot of info we don't.
The guy is on a very precarious edge. He tips forward and improves his offensive skills some, and he's a really valuable guy. He stays roughly as good as he is now, and we will want to let him go when his rookie deal is over, because he won't be worth mega-money, and he will be able to get mega money from somebody.
My guess is this boils down to judgements on character issues, work habits, capacity to learn, and what Stevens wants his centers/PF to be able to do. Maybe the C's don't see Horford as a good fit with him.
The 2017 pick might land an all-NBA player, and it might not. Ainge has to take those chances whenever he can, because the chances of a championship without a 1st team all NBA level player are quite slim. You can't average it out by saying it's only a chance. We only win by taking the chance and being a bit lucky.