I'm not patio11, but my guess (completely speculating here, of course) would be that "Merry Christmas" sounds more like something someone you know in a casual c

I'm not patio11, but my guess (completely speculating here, of course) would be that "Merry Christmas" sounds more like something someone you know in a casual context (friends, family, etc.) would say to you, whereas "Happy Holidays" is used more by companies, which tend to try harder to be politically correct. When users see "Merry Christmas" rather than "Happy Holidays", they might be more likely to just read the subject line and open the message without checking the sender, assuming it's from a friend. "MC" also sounds (to me) more like an email that is being sent with a specific purpose (as opposed to just a reminder that the company exists), e.g., maybe the company is offering you free bingo cards as a gift.