Despite being a tantalising prospect, Simmons has a massive hole in his game - no jump shot. Not even the kind of lack of a shot where he'll miss mostly - at LSU he would literally pass up wide open mid-range jumpers.
In today's NBA, that lack of a shot is incredibly problematic, especially on a team like Boston which likes it's 4's and 5's to spread the floor as much as possible.
Compare this to Fultz, who has no gaping holes in his games, but rather areas for improvement. He is far more dynamic scoring the ball, WAY better shooting the ball, and their defence is quite comparable. Simmons is not a particularly good defender in a similar way to how Fultz isn't a particularly good defender. They're both dead on average, but can look quite good when they're engaged, as well as having all the tools to be good defenders.
Regardless of this, Simmons has a far more worrying injury history and a very questionable lack of skill (shooting), whereas Fultz looks like he'll be an absolute beast.