Because the United States is a Christian country and was founded upon Christian precepts. Does that mean you yourself have to be Christian? No, but you still have to accept the larger picture.
Well, considering that the first amendment is all about separation of church and state, I have to doubt that its founders had a Christian agenda in mind when coming up with how they wanted their country ran.
Anyways, I'm Canadian, so while I did vote 'neither', I technically have no say in what's printed on the ugly green bills. However, it does annoy me that "in all our son's commands" and "God keep our land" is in the Canadian national anthem, considering that they were only added in the English version. The original French version of the anthem contains neither of those two phrases. But whatever, I don't hear it nearly as frequently as I spend money, so I guess it's not worth complaining about.