住職(じゅうしょく)が耳(みみ)に般若心経(はんにゃしんぎょう)の文字(もじ)を書(か)き忘(わす)れてしまったからです。

To save Hoichi, the chief priest carefully wrote the characters of the Hannya Shingyo (Heart Sutra) all over his body. By doing this, Hoichi's figure was supposed to become invisible to the ghosts. The priest strictly told Hoichi, "No matter what happens tonight, you must never utter a sound." However, perhaps because he was in a hurry, the priest forgot to write the characters only on Hoichi's ears. When night came, the ghosts came to call Hoichi, but his body was nowhere to be seen. The ghosts found only the remaining ears and took them away. Thus, Hoichi's life was saved, but he lost his ears.