Unpublishing pages isn't nothing -- disabled pages can't interact with fans or be interacted with, which is the main reason why a company would want to have one. But maybe they're holding off on deleting it because they might later realize it's a useful (and cheap and efficient) channel for outreach.

edit: or maybe disabling (but not deleting) is the best way to make sure the Tesla/SpaceX pages don't get name-squatted when they are released?

I think that's exactly why they went with unpublishing the page, rather than deleting it. They would lose all their followers and data, which is the hardest thing about leaving a platform like Facebook - the historical context you leave behind.

This also means that the data still remains available to Facebook as well, so while it isn't nothing, it's not nearly as dramatic of a statement that Elon originally made.