He doesn’t fit with Devin Booker
The biggest question with any star player(s) that the Suns acquire is how they will fit with Devin Booker. It is highly unlikely that they will acquire a player using any mode who will be as good as Booker and thus they need to make sure that Book remains the focal point of the offense.
Isaiah Thomas understands himself to be a top-flight point guard and needs the ball in his hands to be at his absolute best. A score first guard, he can put points on the board like the best of them, made most impressive by his diminutive height.
The problem with that in Phoenix is that his ball-heavy offense would take away from Devin Booker (and Josh Jackson for that matter) and lesson their impact on the court – and I would be hard-pressed to believe that in the end anyone could honestly say that Devin Booker will NOT be a better player than Isaiah Thomas at some point very soon in their respective careers, so his skills must be taken advantage of before all others.
If signing Thomas actually does take away from Booker in the back court, then one must question whether or not he is the right fit for Phoenix.
Scoring near 30 points per game is an incredible feat. But Thomas has only averaged over 6 assists per game twice in his seven seasons. Granted the Celtics won 53 games the year Thomas averaged 28.9, and could have potentially made it all the way to the NBA Finals in 2017 had he not been injured in the playoffs, but the fact remains that during the regular season, when he hovered around 30 points per game, no other player was close to 20, and power forward Al Horford actually only averaged .9 assists per game less than Thomas, at 5.0.
Point guards should be distributors first. Scoring of course needs to be an important component to their game, but with a team like the Suns who already have three players who can score at a very good clip (Booker, Jackson, and T.J. Warren), having a point guard who might take away from their offense might only hinder them, and not help.