An answer posted to a community wiki question will also be community wiki.
What is the reason to make answer community wiki automatically in this case?
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The point of community wiki is to make more editing happen, and according to this, "The intent of community wiki in answers is to help share the burden of solving a question", while "questions rarely, if ever, need community wiki" and "Community wiki is for that rare gem of a post that needs true community collaboration."
So you would only make hard-to-explain questions Community wiki so people collaborate more on the answers.