Answer:
During a particularly narrow window (about 3 years) of Shaq's prime, he was the most dominant player in the game. But during all the other years of his 19 years, Shaq was often not the best player on the court in a playoff series and was frequently outplayed at his position. Players like Ewing, Olajuwon, Robinson, Duncan, and Malone frequently got the best of him with positioning, toughness, and better use of their teammates.
He was always better at offense than defense and until the Lakers turned the corner by adding key role players, he was getting beaten like a red-headed stepchild, every year. Not narrow losses either, but sweeps. Full on, humiliating sweeps where he'd put up 30-10 but give up even more on the defensive end because he wouldn't run back and go 5/15 at the line.
He was a selfish, self-promotional player, constantly putting himself before his teammates. He burned bridges on almost every team he was with. When he left, he'd often speak ill about his former teammates and coaches, as if they'd faulted because he wasn't there anymore. His lack of dedication and professionalism likely cost the Lakers the title in 2004, where he delayed his surgery so that his recovery would require that he missed games during the season, then didn't ever fully reach his playing shape. Fittingly, he averaged 6 rebounds per game less than the starting center on the Pistons, Ben Wallace, a player 4 inches shorter and about a hundred pounds lighter.