Because the island is moving west. In 1979 there was a long walk from the lighthouse to the beach. When it was moved hardly any beach was left. This happened to the prior lighthouse also. To see this, go to the surf and look for a black line. That's the organic matter from where the sound was. You may find some oyster shells stained black from the same organics. Barrier islands are a dynamic system and always changing. Hatteras Island used to be a round profile until the government decided it needed dunes in the 1930's. FYI Those shells your finding are mostly fossils.
The lighthouse was in danger of sinking into the ocean. So it needed to be moved.
Look at there website, for that information
Thanks! Your answer is awaiting moderation.