Nightmares
The most obvious reason was the nightmares.
John ran from them, literally and figuratively. Kept walking, kept working, kept going, kept distracting himself until he reached the point where he couldn't anymore. Sometimes he felt guilty for pushing Sherlock on the same topic. Other times he didn't.
Moving into 221B Baker Street had been, easily, the best decision of his life.
The cases, the excitement, the work, Sherlock...
They were the best distraction he could ever have asked for.
His therapist used to accuse him of torturing himself. It wasn't like he could control them – the nightmares. If he could have stopped them, he would have a long, long time ago. He wouldn't wake in a pitiful fright shortly after each time he closed his eyes. He wouldn't wake up and shove his face into his pillow to stifle his heavy breathing, or, on worse occasions, his sobs. He wouldn't fear waking his possibly-sleeping flatmate.
He would feign sleep often. He would lie awake most nights, after nightmares, entirely unaware of the Consulting Detective glancing at the stairs leading to his bedroom.