why is it merry christmas and not happy christmas

The phrase "Merry Christmas" is actually a contraction of the phrase "God bless us, every one," which was popularized by the famous Charles Dickens novel "A Christmas Carol." The word "happy" was originally used to describe people who were fortunate, while the word "merry" was used to describe people who were cheerful and lighthearted. So, when people started wishing each other a "merry Christmas," they were wishing each other happiness, good fortune, and good cheer.