Probably for the holidays.
>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."
http://www.npr.org/2016/12/20/506215632/how-red-and-green-be...
Given where HN is based, and that the holiday is fairly well known even among those that don't celebrate it, I assumed it was obvious - my mistake.
The question may even be referring to the rationale of having HN decorated for Christmas at all, given that it's not universally observed.