Dueling is barbaric and can often start blood feuds: a cycle of violence. It's also a bad way to settle a dispute. The person best at shooting others has no correlation to how well their position was or whether the fault or insult should stand. Also killing for an insult is juvenile. There's lots of other real reasons history outlawed it, as well.
By the end of dueling, people weren't even shooting at each other. It's braver to be shot at and the point of dueling is to appear brave. They would fire into the air as if they were saying "I'm so brave I'm not going to shoot at you". It was dumb, so we stopped the charade.