Well, I'm no expert, but common sense dictates that since it will be the first time doing a Halloween overlay to SM, it just might take a little longer than the one for the HM that they have done for numerous years now.
Because they have never done the Ghostly Galaxy overlay on SM before. Anytime you do a new overlay for the first time it will take longer to set up. They've got HMH down to a science since they've done it many times before. I read somewhere that if they do the SM overly again next year, it should take no more than two weeks to set up instead of the several weeks its taking this time.