One possible reason (I don't know either) would be that an event MUST be held on the date specified, whereas a hidden cache can have its hidden date set to whenever the CO chooses. For example, there have been several caches hidden with hidden dates listed as being before geocaching started (they seem to have stopped that though). Therefore someone who wanted the addons would just have to set their hidden dates to the dates they need, rather than when they actually hid the caches.