Hardens phlegmatic style and body language combined with his tendency to slowly bring the ball up and dribble the game clock down to 10 seconds before running first action are probably the biggest reasons why he keeps falling flat in the playoffs. The latter can be so detrimental to your team if your first action keeps failing constantly.
Clippers didnt get any easy looks in game 5 yet they never really sped up their game to break the structure of Mavs defense or to allow for more actions. Harden made sure the pace stayed lethargic, even when they were down 20.