Because forum repository is very often used as shared -> Commenting module should check during installation if "Page Comments" forum already exist or not.

So instead of bootstrap "Page Comments" forum (src/main/resources/mgnl-bootstrap/commentingforum.pagecomments.xml bootstrap file) into forum repository automatically, add extra install task into CommentingModuleVersionHandler which will create "Page Comments" forum only when it doesn't exist. This will allow install commenting module on environment with shared forum repository.

For more details see SUPPORT-3875 or https://wiki.magnolia-cms.com/display/WIKI/Setting+up+a+Jackrabbit+Clustering?focusedCommentId=72289219#comment-72289219