Because of the danger of the dinosaurs escaping the island and becoming a reemerging predator in the wild, the scientists integrated a fatal weakness in their DNA, as explained by Dr. Wu:
"We don't want them to survive in the wild. So I've made them lysine dependant. I inserted a gene that makes a single faulty enzyme in protein metabolism. As a result, the animals cannot manufacture the amino acid lysine."
(Crichton, Jurassic Park, Google Books)
As Wu explains, without the ability to manufacture lysine, the dinosaurs will die after about twelve hours, thus preventing them from spreading. Since dinosaurs are assumed to have primitive brains, they should be unable to identify their missing nutrient. However, at the end of the book, something unidentified is traveling across the mainland, eating certain crops like agama beans, soy beans, and chickens, all of which are rich in lysine; the implication is that some dinosaurs made it to the mainland, and now understand which foods they need to eat to survive.
He engineered them to be unable to manufacture the lysine they need. To stay alive the dinosaurs need to eat the special lysine-rich food given to them in Jurassic Park. Lysine is found in meats, milk, soybeans, lentils, and spinach.