How do Toronto and Phoenix get on that list and Boston doesn't?
Talent. They have more high level than Boston does.
(1) Phoenix has two above average starters - Bledose and Markieff Morris
plus Brandon Knight who got some All-Star consideration last year
(2) Toronto has two recent all-stars - DeRozan and Lowry
plus Toronto has an up and coming center in Valanciunas who is rated fairly well
(3) Boston has zero above average starters. Boston has zero average starters. Every player is either a below average starter or a bench player.
There is very little on the Celtics roster to convince a player he'll have the help he needs to win if he does join the team. Those are not perfect but they have more quality talent & are further along than Boston is.
Plus, I think Phoenix gets bonus points for weather and style of play.