Parkour FOCUSes on efficient movement. It's all "about getting there as fast as you can." Competition changes that main point of Parkour from efficient for you, to flashy for the Judges!
Parkour is a self-driven activity; no one forces you to take part, but if you are interested, there is a community to help you accomplish your goals. Competition removes that community and leaves begginers on their own. Traceurs need others to help progress.
Training for Parkour is a delicate balance of mental strength, and understanding your physical capacity. Competition removes the understanding and rather focuses on doing what you SHOULD do; not things that benefit you, but things that will make you win a competition. This is what causes INJURIES!