The reasons for it have almost certainly changed over the years.
They will almost certainly continue to change over the centuries to come.
The traditions involved in celebrating Christmas also vary by region and country. For instance, Kentucky Fried Chicken's Japanese branch managed to foster a bizarre tradition of eating KFC on Christmas, to the point that it is now almost impossible to eat at any KFC in Japan on Christmas without a reservation.
Japan has a small minority of Christians in the country (nearly 3%), mostly Roman Catholic. Despite this, Christmas is HUGE in Japan, mostly centered around Santa Claus.
Turns out, Christmas as a gift exchange holiday is a extraordinarily successful meme, because it's a huge end-of-the-year economic boost, right when the economy needs it most. Natural selection applies to memes the same way it applies to genes; cultures that celebrate Christmas or something similar tend to do very well for themselves, while cultures that don't have to find some way to make up the economic boost or else just carry on as usual through the days between the Winter Solstice and the New Year.
As a result, every culture in the world that can justify it to themselves wants to share in it, because of that incredible economic shot in the arm every year. Why the fuck wouldn't they? It's the best thing ever!