Williams started out last season slowly (and not in great shape) but played his way into shape and due to injuries got more run as the season wore on. And he showed potential in those minutes — he didn’t score a lot (4.7 per game and he has no shooting range to speak of) but rebounded pretty well on both ends of the floor and played within himself. He’s no lock for a long NBA career, but you can see potential in him.
For teams looking for another big, most would rather have a young player with potential than a veteran on the down side of their career. It’s why he will be off the market pretty quickly.