They lost the game because they were outplayed in the second half, physically.
Because the bench scored a total of 4 points.
Because the starters had a total of 10 assists. (Harden had 8 himself.)
Marcus shot the ball 22 times, at a horrible percentage, while Gordon Hayward shot the ball 16 times at near 50%.
Harden saw he wasn't having a great night, for what ever reason, and handed the keys to the car to Westbrook. (Or should I say Westbrook took them)
Shooting percentage, and assists (assisting in easy baskets) mean something. There are only so many shots in a game. Knowing when to stop trying to put the square peg in the round hole, and learning to decoy yourself by finding the man with a better percentage shot also means something.