The roster construction is flawed.
Teams focus on several main things:
-forcing the 76ers into a half court game
-taking JJ away as an offensive option.
-switching offensively onto the 76ers bad defenders to score at will
-take advantage of the 76ers poor decision making, causing turnovers
Combined with limited depth, poor outside shooting, lack of rebounding, lackluster foul shooting and a general attitude of "we'll win it in the 4th quarter" as Simmons stated after the recent loss to Atlanta makes this a team whose ceiling is low.
Brown seems unable to reach the players to give consistent effort and fix these years-old issues. I do not blame him as much as the front office, but he likely will be the one who pays the price first.