What I find kinda strange in American Culture (as an American) is that the rest of the pop music tapes are pretty much just period music that no modern kid would recognize.
The Christmas tape is a 100% known landscape. We let hits die but Christmas music is eternal.
One could argue that the whole point of traditions like Christmas is to have comforting, repeated experiences like listening to the same music each year, something to ground your mortality in timeless rituals that started before you were born and (you hope) will continue after you die.
That said, there is lots of Christmas pop that doesn't make it into the canon. I have no recollection at all of the first song on the Christmas tape, and i was around then (and going to K-Mart with my mom).
I think it's a combination of copyright and licensing.
The pop music they use on these tapes are cheap to license --- they weren't even popular when they were contemporary. They were just generic cheap music.
For Christmas, there is a set of Christmas music that just gets played every year and is licensed in blocks that are fairly cheap. That's why you hear a lot of "traditional standards" and not much else.
I'm not sure on the accuracy of that XKCD graph. Last Christmas by Wham! is noticeably missing (released in 1984). Also All I Want for Christmas is You, released in 1994.
The Christmas tape is a 100% known landscape. We let hits die but Christmas music is eternal.