Tradition. Either one is grammatically correct, but when most people say "Merry Christmas" and "Happy Birthday", then that is what you expect to hear. In fact, in some places, people say "Happy Christmas". "Merry Brithday" is also used at times.
Just as in your question !They tend to say "Happy Christmas" instead of "Merry".Merry Christmas
You say ' Merry Christmas' in the UK.
merry christmas
"Merry Christmas"
merry christmas
Merry christmas
Merry Christmas
merry Christmas= Christmas aashamsakal
Hanukkah is a holiday, not a language. You cannot say "Merry Christmas in Hanukkah. That's like asking: "How do you say Merry Christmas in Easter" I think he means how do you say merry Christmas in Hebrew
Merry Christmas
Merry Christmas
Merry Christmas in Cree