Allowing him to believe HE was that cause of her death fuels his emotions, his anger and his despair. His mother's death pushed him closer to the Dark Side, as it made him feel helpless. Making him believe he was to blame makes it that much worse.

She died, as they said, because she lacked the will to live. Everything she believed in, everything she loved was now dead. The Republic, Anakin. All were lost. The twins existed, but also existed as reminders of what had happened. She was heartbroken, and had nothing to live for. She just gave up.