Because he's lost it, his athleticism and pop are gone from his knees. He couldn't rebound anymore last time he got on the court in the NBA, couldn't really do anything.
Actually, in his last full month he averaged an amazing 7.4 points on 62% shooting in 10 minutes, with an equally amazing 3.4 FTAs per game. He never shot less than 50% in a game and had a game where he went 9-10 at the line in 10 minutes (got to the line in all but 1 game). Obviously he wasn't healthy enough to get real minutes (plus he was fighting with Gasol/Randolph/Arthur/Battier) but still, there were much worse end of bench players.
Unfortunately his game never held up after that last injury, even to this day. It appears that was the straw that finally broke the camel's knee. Almost 0 chance he ever makes it back into the league.