Because he can't handle double teams in the slightest and his vision isn't great. He also doesn't establish position and sees himself getting pushed around on the low block. Dude doesn't set screens, get a small guy on him, and then fight to establish position.
So it takes 5-7 seconds to even get him on the block, another 3-4 seconds for him to father himself and space the floor, and this isn't counting any time to bring the ball down. Then the second the double comes, it's a turnover or forced shot.
I see guys like Steven Adams, far less talented, but they understand that the battle of the big men is in fighting for your spot and holding it. Whiteside DOES NOT DO THAT consistently. He just doesn't.