Maybe big organized religion fell out of style because crooks were using it for personal gain, and to manipulate the masses. In this future, maybe people have traded in big religions for individual spuritual relationships with what they call God. Maybe in this reality the world rejected classical faiths when they found the Traveller? Maybe I this reality Christmas had become a day so dominated by marketing and consuming that it lost all of its associations with religion. After the collapse there just wasn't room for it, and without the sentimental religious attachments just faded with time. Maybe in this reality Buddhism became the dominant religion? Maybe in this reality Bungie didn't want to create a world divided by religious beliefs. Instead they chose the traveller as a metaphor for all forms of religious worship? Maybe in this reality there is Christmas, but it takes place wherever your guardians call home whn you aren't logged on. Maybe in this reality Starbucks took over the world and finally won its battle against God.