Why do Americans always say happy holidays in place of merry Christmas?

I always watch commercials and other things and all they say is "Happy holidays" without ever saying merry christmas.The same applies to when people see each other,they say Happy holidays.How many holidays at all do workers get in December especially during the Christmas period?Its only on the 25th and perhaps 1st of January to,but all they say is happy holidays.Can someone explain the rationale behind this greetings or statement and why dont people just say Merry christmas as most Saxons do?