Turns out, a completely different volcano destroyed Isla Sorna right before they wanted to move the dinosaurs back there. Nature finds a way to fuck dinosaurs over again.
You got me, man. When corporate hack who's obviously going to turn out to be evil started talking about moving them to a new island, I kept expecting Claire to give the audience a bit of expository dialogue about why they can't just move them to Isla Sorna/Site-B, since it was turned into a dinosaur nature preserve protected by the military anyway, and he would say something like "Because the island's plants caught a disease that gave all the herbivores explosive diarrhea, which in turn gave all the carnivores who ate them explosive diarrhea, so the Navy just said fuck it and nuked the sight from orbit rather than deal with the cleanup. It's the circle of life. But you should have known that already because you're the Save the Dinosaurs lady."