So? He stays pretty busy with Solid / Inrupt and his other advocacy (e.g., the OP article...)
Perhaps he sees the personal blog format as a bit egotistical or otherwise not fit for purpose. Or perhaps he just isn't a naturally talented writer. Its not as if he's shy about sharing his thoughts outside that.
Also worth noting that he and Bruce Schneier, who is a much more prolific writer, share many of the same views and partner together at Inrupt.