I am still speculating but...

1. You have to push out an update to re-issue the expedition(s) on a shortened timeframe. While the development cost itself may be minimal, there's still a cost associated with doing the release on various platforms.
2. I assume expeditions cause a spike in usage. Since this is a multiplayer game, it has to be hosted somewhere and that isn't free, especially when traffic increases.

Both of these may be very low cost but it still may not be worth it for them to do it. They continue to release new content for free, which is great, except it's also not a sustainable business model. They probably have to keep it as lean as possible.