Why was it difficult for American forces to defend themselves against Japanese fire?

During the invasion of Iwo Jima, approximately 70,000 American troops arrived to the island. However, a force of around 20,000 Japanese soldiers had entrenched themselves in the mountains. Moreover, these soldiers had built tunnels throughout the island. Therefore, when American troops arrived, they encountered very strong resistance from the Japanese. It made it very difficult for Americans to protect themselves against Japanese fire.