Leonardo's double hull design merely appears as a drawing on the side of a page in the Paris Manuscripts. Drawn in a manuscript that describes war machines, the double hull was probably a way of protecting against Leonardo's own designs, as a second hull provides extra protection from outside damage, such as enemy attacks or submerged objects. However, the double hull would not appear in an actual ship for another 370 years. As for the reason, double hulls are more expensive and very difficult to build. Going by deductive reasoning, without modern technology, the double hull likely only makes economical sense in very large and expensive ships, such as the Great Eastern. Also, materials like metal most likely made building the necessary parts feasible.
Source:
Innocenzi, Plinio, and Edward Burman. The Innovators behind Leonardo: the True Story of the Scientific and Technological Renaissance. Springer, 2019.