Maybe the ad agency gets a little stir-crazy toward the holidays, and don't feel like coming up with anything new.
The only thing we've been able think of is a commercial where the candies can't think of any new ideas and decide to duck out of work early for the holidays. You should probably just use that same commercial instead.
See ya after New Years!
-The Ad team."
Or maybe that commercial is too good, and no one can top it. Like some kind of advertising zen riddle, it drives haggard creative teams year after year to try and find an idea that's just as cute, quick, and funny as what they've already got. Even now, some ambitious young ad exec is collapsed on his desk, surrounded by crumpled drafts.*
*And you just know the guy who wrote that commercial comes back every year to lord it over them.