Because he's a terrible basketball player?
Look at his numbers for the season.
37.3% from the field (47% true shooting)
The league average, by the way, is 44.7 and 54, respectively.
27.1% from 3-point land (on 7.6 shots per 48 minutes! Twice the league average! He takes a lot of 3 point shots and has no prayer of hitting them)
He doesn't get to the line (and is a bad free throw shooter anyway, at 70% for the season).
He can't rebound.
He can't pass the ball (his assist to turnover ratio is pretty much 1:1, which is awful)
He's a bad defender
Why on earth could he have possibly helped them?