He says he waited 21 years because he didn’t want to jump on any exploitative bandwagon.
It’s been 21 years, so it’s not like exploiting the story for personal gain. If I am, I’m doing a lousy job of it – I should’ve rushed something into print a year after the crimes came to light. I just didn’t want to be a part of the scandal, the sheer feeding frenzy. I wanted to tell a story that I thought was very unusual and compelling, and I wanted to do it my way. So I took my time, and when I finally got around to doing it, I did I the way that I’d always envisioned, and that was important to me. (source)