It’s become routine for online atheists and atheist organizations like The American Atheists to piggyback on Christmas (and perhaps, to a greater or lesser extent, other traditional Christian holidays).
The reason many would likely employ to explain this behaviour is that of publicity and exposure. Atheism, particularly online, makes a noise, but it remains a vocal minority. It also remains a minority in practice, which likely testifies why a maximum of 10 000 people turned out at a Reason Rally in 2012, and that such a figure was dubbed “the biggest gathering” of non-religious people in American history (1). This figure decreased to 5000 as of 2016.
However people view these statistics, most would agree that they are quite small, especially if we compare them to religious events such as in the 6 000 000 attendees in Manila who gathered around the Pope when he visited, or the 30 000 000 Hindus who gathered during the Kumbh Mela pilgrimage.
Apart from several atheist authors who have sold their books very well, there is not too much else in the way of publicity. Consider, for example, a documentary featuring Richard Dawkins and Lawrence Krauss, two big and influential names within contemporary new atheism. Their documentary appears to have performed rather badly grossing no more than $14 400.
Perhaps this would explain (or constitute one of several reasons) why atheists piggyback on the Christmas bandwagon.
1. Winston, K. 2012. Atheists Rally On National Mall; The ‘Reason Rally’ Largest Gathering Of Nonbelievers (PHOTOS). Available.