It's one thing to not originally have a Nintendo character on the shortlist, but it's quite another to not add them to the list of possible candidates upon seeing their massive popularity quantified via the ballot, which is why it exists in the first place, to provide Nintendo with information regarding the desires of the fanbase.
Nintendo's goal first and foremost with this DLC is to have it sell as well as they can. If they're going to omit highly popular, perfectly feasible characters who have a clear potential market evinced through the results of the poll, they're being nothing short of counterproductive. Recency and promotion has it's place, but characters who will sell, regardless of if they're not "current" do as well when it comes to buying characters on an individual basis and not as a package deal.
When you buy the initial game you don't have a choice as to which characters come with it, so they can stick as many partially promotion-related, partially recency-related additions as they want, but when you're doing nothing except for selling a character based on who that character is, you want to go for what you know will sell. At that point it matters a lot less when your last title is as long as you'd turn a respectable profit, which obviously the most requested characters would.