Because at the moment, you only get shown this particular entry when you first join a community, and can't then find it later very easily. In most communities, it may also be the sticky entry, but not necessarily. Some communities may have a "posting rules" entry, which is used for this, and a separate sticky entry, which links to a posting rules entry, a FAQ and a contact the maintainers entry, for example. If they don't manually link to the posting rules entry from somewhere else, the posting rules entry is only shown when you join the community, and can't then be found again by anyone other than the maintainer, unless you leave and re-join just to get shown it again.