I think cylinder hit the nail on the head, with his remark, about it bringing office culture to the front - if you don't enjoy working with the people at your job, it sure isn't looking bright for the Christmas party either.
Also, a Christmas bonus is waaaaaay more expensive than a (nice) Christmas/office party has to be (unless you rent a fancy venue or something like that). It would feel a bit weird to get a $30 dollar "bonus", at least I would rather pool that together and make some enjoyable memories with people.