Two reasons I can think of: Facebook does incremental rollouts, so they don't want to announce features that everyone doesn't have, plus Facebook users are extremely hostile to change (that they know about) so providing a list of changes is counter productive.

That might be true but I do recall that last "cover" update was announced to all but rolled out incrementally as well. The mentality issue might be the true root cause but perhaps if they did more announcements people might get more used to the idea of things changing.