>this friendly, fat Santa in these bright red robes, which, I don't think is a coincidence, match the color of the Coke logo — this really took hold in American culture."
Because it's the day that we celebrate the birth of a man who was nailed to a tree over 2000 years ago for saying how wonderful it would be if we could be nice to people for a change. Even though today isn't his birthday.
There are large parts of the world where Christmas isn't celebrated, so it's entirely plausible that not everyone recognizes these cultural references.