Because we're the people funding them, so we can't be trusted to actually stop giving our money to users of slave labor. But obviously ending slave labor is more important than eating cheap chocolate, so we need to find an alternate way to stop it. What other way could we use but to put a ban on anyone caught using slave labor, to the point where it's finally enough to prevent it from happening?

If a "regulation" is just a fine that doesn't outweigh the profits from the act it's meant to be regulating, it's not a regulation, it's just the government taking a cut of the dirty profits. We need real regulations - forfeiting of all annual profits any year they were found to have used slave labor, for example.

Any company found to be profiting from dirty tactics needs to be swiftly hammered into the ground by every regulation we can throw at it. No product is worth the cost of human lives.