Community Wikis are answers posted originally by a single user that are intended to be maintained and modified by the community to add additional information. The original poster (and all who edit the wiki) do not earn any reputation for votes received on them.

The tooltip for the community wiki checkbox states:

Marking an answer community wiki encourages others to edit it by lowering the reputation barrier required to edit. However, you will not gain any upvote reputation from it. This cannot be undone.