Because ninjas are all about darkness and secrecy. A ninja loses a substantial amount of his power when there is a crowd of people watching.
Also because the metaphysical nature of ninja powers requires a high level of concentration to achieve mastery. A hollering audience dissipates your chi. And because, as already observed, it is hard to sneak up in an octagon.
Just like how a pirate is more powerful than a ninja if they are fighting at sea because the ninja is sober and disoriented from the motion, but a ninja is more powerful than a pirate on land because the pirate is drunk and disoriented from all the levelness. Unless the pirate is also a vampire. Vampirates can kick anybody's ass. And they have superpowers.
What are we talking about again?
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