The people on the mainland didn't lose the technology. They lost the interest. Ancient Mediterranean was full of naval cultures. But when the Roman Empire decl

The people on the mainland didn't lose the technology. They lost the interest. Ancient Mediterranean was full of naval cultures. But when the Roman Empire declined and fell in the west, maritime trade declined as well. Then Arabs conquered North Africa, turning the Mediterranean into a battleground. Nobody cared anymore about some remote sparsely populated islands.