A pick is more valuable in a trade than a player. Having the pick roll over raises its expected value pre-season and gives the Celtics more time to trade it.
FWIW Memphis is 5-8 with a -7.52 SRS (26th in the league) and an expected win/loss record of 3-10. Second worst net rating in the west, only better than the Warriors.
They're still on track to get a top 6 lottery pick.