Continued Hip Injury
The most obvious reason here is Isaiah’s continued hip problems. It all started towards the end of the 2016-17 season with the Celtics. Isaiah played through it in the playoffs, even though he probably shouldn’t have. There was even some back and forth after the Cavs trade between Isaiah and Celtics’ ownership about the handling of his hip. Either way, Isaiah put forth a valiant effort in the playoffs to keep the Celtics in it while dealing with the hip injury (and the death of his sister).
It would have been one thing for the hip injury to be isolated to the end of the Celtics season, but it lingered. His recovery from the injury followed him to Cleveland, and although he still seemed like he was trying to regain his former explosiveness, the injury seemed behind him.
That wasn’t the case, as the Lakers are now without Isaiah for the rest of the season. Isaiah will be turning 30 years old next February, and teams are going to be very hesitant to invest multiple years and multiple millions of dollars in the small point guard. His hip problems will just be an added concern in his later years.