1. Do you see any asurans or dwarves doing snowballs ?
2. Charr would probably prefer to chase balls of wool instead of snowballs.
3. Most races probably think of snowball fights as kiddish .
4. I could argue a theory that the only reason norn join in - the fighting requires a small amount of skill in taking aim and throwing so norn may look upon this as a form of contest to show you skill in aiming and throwing rather than a kids game.
Too early in the morning to think of more but you get the general idea