Google says it's this, but who knows if it's true.

>Lore has it that the very term Canadian tuxedo comes from a Bing Crosby episode at a Canadian hotel in 1951. Crosby and a friend were reportedly denied entry to the hotel because they were wearing jeans, which inspired Levi's to design a denim tuxedo for Crosby so that he could have a formal denim option.

I think, if that story is true, that means it just became one of those things where the origin was lost as people repeated it more and more.