A: Because the dolphins can't be sure they're actually jumping to safety.
It's an often-asked question, because it seems to have an obvious answer. Clearly, the dolphins in Taiji could just jump over the nets that keep them trapped in the killing cove and swim to safety. But the dolphins don't know that. As Save Japan Dolphins points out, the dolphins can't see from above, thereby making it impossible for them to know for sure that there isn't more danger lurking on the other side of the net. And, according to Paul Watson, founder of the Sea Shepherd Society, dolphins won't jump somewhere unless they know what awaits them on the other side.
Jason Bruck, a dolphin researcher at the University of Chicago, takes it one step further: he says the dolphins don't perceive the net as a trap they can escape at all, the way humans do. "Dolphins have a different perceptual understanding of the world than we do," Bruck told The Dodo. "They don't perceive... how to get out of [the nets] without some form of training, which is the same reason a dolphin can be trained to jump through a hoop but won't do it naturally."
But, Bruck added, researchers have not reached a consensus on this. "Stress could also be a factor preventing the dolphins from thinking through the problem," he said. "The key is that the nets are unnatural to them, and they have not evolved (long term) or innovated (short term) a solution that would allow them to escape."
So, although dolphins do understand natural boundaries, like the shoreline or the ocean's surface, they are unfamiliar with artificial demarcations -- which is only one reason why they should never be trapped in any net, line, or cove.