The problem with traditions like this one is that back when they started they didn't have the unsustainable impact that they have today. The human population was a tiny fraction of what it is today, and the ability to "harvest" these animals was a tiny percentage of what it is today. Back then there were no global fleets, little in the way of international trade, and a much smaller demand. Basically, if you couldn't catch the animal yourself, or arrange to have it shipped from someone who could, you didn't eat the stuff.
As for why people eat it ... because it's a statement of wealth and prestige. Shark fins add nothing to the flavor of the soup ... that comes from other ingredients. Shark fins only supply the texture. But because back when it started, only wealthy people could afford the fins and a chef with sufficient skill to prepare it properly, it became a status symbol. That's all it is today ... which is why it's so difficult to get people to stop using it.
... Bob (Grateful Diver)