Possible Duplicates:
Should CW activity be rewarded similarly to normal posts?
What are “Community Wiki” posts on Stack Overflow?
Why don't we get reputation for answering community wiki questions?
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Sign up to join this communityPossible Duplicates:
Should CW activity be rewarded similarly to normal posts?
What are “Community Wiki” posts on Stack Overflow?
Why don't we get reputation for answering community wiki questions?
Because the answer is "owned" by the community and not any one individual (even more so than usual).
Making an answer CW is supposed to encourage others to edit it and make it "the" answer to a question, so then the problem would become - who gets what proportion of the rep from up-votes (or loses what proportion from down-votes) if the answer were still "owned" by the individuals.