It's pervasive ubiquitousness during the holiday season and the impression that singing the song will literally take twelve days to complete are enough to land it on the list. But the real reason why it's here is best explained by my favorite Executive/Action Transvestite:
That “Partridge in a pear tree” song, the only bit we like of it is that, ( singing )"Five gold rings!" People go berserk at that point! People come running in from other rooms, ( singing ) "Five gold rings!" The rest of it, we don't know; above that, it's just, ( singing, uncertain ) "Twelve… monkeys mating, eleven… donkeys dancing, ten pigmies farming, nine… socks… a swimming… ( certain ) five gold rings!" This is a human thing. We only like to learn a little bit of a song. We can't be bothered, yeah.