A part of the crew, a part of the ship.
Because he's becoming part of the ship.
The apparent death of his son Will Turner caused Bootstrap Bill Turner
to sink into a state of madness, so much so that the process of
assimilation into the Flying Dutchman accelerated, and Bootstrap's
humanity began to wane. When Elizabeth Swann was thrown in the
Dutchman's brig, she was able to coax Bootstrap into a conversation,
though it was clear that he remembered little of his life. He still
recognized the name of his son, however, and held on to the hope that
William had promised to rescue him. When he realized who Elizabeth
was, he lost hope for his own redemption. Bootstrap told her that Will
would have to choose one of them and jokingly stated, "I wouldn't pick
me." Elizabeth tried to wake him after he had merged with the ship
again, but he appeared to have completely forgotten their prior
Some conditions of years of servitude under Davy James is that, at some point, one begins to grow barnacles and sea-creature attachments and then later, that crew member eventually becomes a literal part of The Flying Dutchman. In Bill's case, the process was accelerated due to his grief and loss of faith, believing Will was was dead.
As we see from the scene above (video), Bill Turner was about to become part of the ship when Elizabeth happens upon him and awakes him. He temporally snaps out of it, walks out of the wall, and talks to her as Bill Turner, but soon after forgets and repeats himself and begins to settle back into the wall of the ship.