Plus the retailers and magazines like being able to sell it earlier, to take full advantage of the December shoppers. They then get to have a further "New Year" issue out just before Christmas too!
although the New Year one doesn't come out until the 22nd December, so will probably not be a priority for many that close to Christmas, be missed due to Christmas and the bank holidays, and end up being purchased no earlier than about the Wednesday, the same as any other issue.
I'd say including the smattering of festive shows in the full week before Christmas instead of including New Years Day was the wrong decision. New Year's Day seems to be only second to Christmas Day for BBC One thesedays, so it seems odd that those big highlights are excluded from the double issue.
Alternatively do a 16 day edition, change the deadline dates and delay the publication date, nobody really needed to buy it last Saturday!