Currently MediaWiki doesn't offer admins a way to send notification to users, a feature that most CMSs out there have.
One way to fix this problem would be to use MassMessage in combination with the user registration process. Imagine that new users would be subscribed by default to a SiteNotices page, automatically or perhaps after with a checkbox added to the registration process, or in the Preferences.
This would be a great interim solution for new wikis. Current installations ould still need to find a way to subscribe existing users, but that problem is out of scope here.