I think because he was already 37 or 38 at that time, and was suffering from injuries, too. There was no point continuing if both his body and mind are not the way they used to be.
I think most devs can delay burnout/leaving the field if they begin viewing programming as a means to an end, e.g. "programming as a way to build their own business," or "using programming knowledge to mentor others," or "using programming knowledge in another domain they're interested in to great effect."