All Saints' Wake has already missed its proper season, so it makes more sense to delay that one event further than to try to squeeze it in ahead of the two other events scheduled to run in December-January and push them all out of time.

Also, even if the Saints' Wake event files were separate from the 6.0 patch, running it as scheduled in the first half of December would mean the event decorations (and music zones) are up in the starter cities at the point where they're potentially being used as an MSQ setting. I'd rather that not be the case, so I'm glad it's been moved away from the new expansion release date.