As others have said, it’s the banality, mawkishness, and general low quality of the music. The repetition just makes it worse.
Unfortunately, my general hatred of Christmas music spills over onto the few Christmas songs that are actually pretty good. Merry Christmas, Baby by Bruce Springsteen is decent, and Joan Jett’s Little Drummer Boy is awesome. But they rarely bother to play that one. Regardless, I don’t want to hear them.
That said, I do like certain traditional carols, like Silent Night, especially when sung under the right circumstances and with the requisite amount of gravity. Actual human carolers standing outside holding candles singing Silent Night = good. Novelty version of Silent Night coming out of the radio while I’m bombing down the NJ Turnpike at 80 mph = atrocious.