Because Nurkic is a great player when he's a) in shape and b) healthy... which is to say never.  Nurkic has a major ego and attitude problem.  He was not willing to play backup to Jokic and made it clear to the office that they should trade him.  Once he hit Portland, how many good games did they get out of him before he got injured?  Two games.  Meanwhile, Jokic was rated 16th best player in the entire NBA.  Plumlee will make a great backup.