The disappearance of the Double Down from KFC locations across Canada last fall, after a one-month trial, was one of the great mysteries of modern marketing.

The controversial and popular sandwich is returning on a limited basis, but one unanswered question remains.

What prevented the chain from making the bunless sandwich a permanent fixture?

After all, the Double Down had been installed on the KFC menu in the U.S., even before a similar limited-time offer even expired last May.

And, more strangely, the Canadian headquarters sought attention for the fact the sandwich would no longer be available after it sold more than one million between Oct. 18 and Nov. 15.

"That's enough Double Downs to stretch across 2,083 hockey rinks or approximately 140 CN Towers from end to end!" exclaimed marketing officer Dan Howe.

So, there had to be a reason why Canada was starved of such a popular product for more than half a year.

Some corporate turmoil might have been to blame. Priszm Income Fund, the owner of more than 400 KFC, Taco Bell and Pizza Hut outlets in seven provinces across Canada, filed for creditor protection in March. None of the social media momentum for the Double Down could save its ship from sinking.

KFC Canada then introduced an entirely different sandwich last month, the Zinger.

But the reaction to a more traditional hot and spicy chicken sandwich, served on a traditional kaiser, couldn't compete with enthusiasm for something that placed the bacon, melted cheese and secret sauce between the fillets.

The return of the Double Down indeed made national news immediately after it was announced on Wednesday morning. Further publicity will be sought on Friday, when the sandwich is made available for lunch at 15 locations across the country, all of which were surely chosen for their proximity to a major television newsroom.

Never mind that the ingredients required to make the sandwich were still available all along. Reportedly, based on some online comments, anyone who asked their local KFC to hold the bun, and add another piece of chicken instead, was granted their wish.

Still, that's no competition for the notoriety that KFC has gained by offering an item that defies the advice of every self-respecting nutritionist.

KFC Canada marketing director David Vivenes admitted, though, Canadians expressed a desire for less sodium than found in its U.S. counterpart. The result is a 10 per cent decrease from the 1,740 milligrams found in the original Double Down.

Will the change be enough to keep the Double Down around Canada for decades to come? KFC still hasn't committed to offering it beyond this summer.

(Photo: Getty Images)