I dont think Ornn has a huge demand to be honest. Pretty sure he's more of an urban legend where rumor has it most legendary warriors get their weapons from.
Also, if you are most familiar with a hammer, you would use your hammer.
Also, based on his kit, most of what Ornn does combat-wise besides basic attacks is not with the hammer. Only his Q really uses it and that isn't hitting them with the hammer. His enhanced W attack (and R I guess) also uses it, but it's still a basic attack.
ALSO, Ornn uses his hammer in the field to craft items. He may be a demigod, but I don't think he wants to carry three different weapons at one time.