Santa Claus says "ho ho ho" in order to promote a joyful, happy and laughter-rich image for young children. "Ho ho ho" is an onomatopoeia, which is a word that sounds like what it means. It is used as the sound that Santa makes because it is one of the onomatopoeia words for laughter.
When a person say's "ho ho ho," the sound is considered more of a deep sound rather than a "ha ha ha." This deep sound of the "ho ho ho" is also more likely to be low and to come from the belly. Santa is a character with a large belly, so this was considered the most appropriate jolly laughter onomatopoeia to choose for him. The sound is also often associated with warmth, old age and even an overweight or large body type.