To me it seems that "Merry Christmas" denotes the act of gift giving as opposed to just some holiday special. Therefore, I think that the "Merry Christmas" subject line makes the consumer feel like they're getting something.
I think you hit the nail on the head. I'm surprised that this hasn't been brought up by a previous commenter. In America, culturally, Christmas is a celebration to receive gifts. I don't see it this way, but on the surface, I think that many do. This is neither good nor bad, but I do believe that's why the phrase "Merry Christmas $NAME" had a higher conversion rate.