1. Mailing lists can be archived easily, and numerous tools for searching them exist.
2. Mailing lists can be indexed and turned into HTML (again, there are numerous tools for this) making it possible to integrate them with other web information.
3. Mailing lists don't require real-time interaction.
4. Mailing lists are accessible to almost everyone with Internet access, and participation in them is possible for people with different mail systems/mail clients.
5. IRC/chat rooms often require special client software, plus an Internet host with special server software and enough bandwidth to handle the traffic. Getting this all to work can be tough.
6. IRC/chat have notorious security problems; it's not uncommon for sessions to be hijacked or used to break into client or server machines.