What else would Christmas cards say other than Happy Christmas or do some in the US say Happy Holidays. I have noticed some in the UK have Happy Xmas they never get used if I have bought a multipack.
Happy Holidays is popular and there are others, such as refering to the Holiday Season. From the last time I was looking at them a few weeks ago, fewer had Merry Christmas on them than not, and many that did had angles as well.