why has his indifference grown to this level?

Gregor first isolated himself, and then found himself isolated and ignored by everyone around him. He became indifferent to everything because he didn't care anymore. Gregor now gladly accepts his isolation. When the door to his room is left closed, this doesn't bother him at all. He has lost interest in his family. Even when the door is left open, he often simply ignores it. Gregor ignores his body as well, letting himself starve to death and spending his days lying on the floor. The apathy of the family has also infected him, and he can no longer be bothered to care about others or himself. When the charwoman attempts to be friendly to Gregor, this only annoys him and he attempts to rush at her to scare her off. His previous desire for human contact has been completely eliminated, though he occasionally takes a vague interest in events outside his door.