cuz most ships only had one layer of hull. titanic not only had a thicker hull, it also had sinking compartments that could hold water and could be closed off to prevent water from getting to the rest of the ship. you see, these compartments and the hull were weakened by a fire that occurred a few days before setting sail, so the iceberg was more easily able to penetrate the hull and the compartments couldn't be closed in time. even without the fire, the compartments weren't a good solution, as they frequently failed in tests.
