Well, we do say "Happy Christmas," although "Merry Christmas" is much more common.
It's just tradition. There's no logical reason. There are just pairs of words that customarily go together. I believe they are called "collocations."
One of the most beloved of all Christmas traditions in the U.S., Clement Moore's poem "A Visit from St. Nicholas" (often called by its first line, "'Twas the Night before Christmas") ends with
"But I heard him exclaim, ere he drove out of sight—
'Happy Christmas to all, and to all a good night!'"
On the other hand, in Charles Dickens' "A Christmas Carol," "Merry Christmas" is used throughout.