It has to do with experience. Ariza does have more talent and athleticism but he is raw. He played only one season of college ball and has been a bench player in the pros so he's still learning the game. Walton has been with the Lakers for awhile and knows the triangle offense. This is why he's on the court rather than Ariza. If the Finals has proved anything besides defense wins championships (again) it's that experience will usually triumph over raw athleticism.
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