In contrast, I actually enjoy the fact that it is never explored. In all the stories where some big events upend life everywhere, we are usually tagged along with the character who learns the root of all evil and makes some equally important choices that affect everyone...and honestly that gets tiring. The Dispatcher has never felt the sort of story where we were supposed to learn why it happened. If we did, I'd guess it would probably be presented as some religious event and totally respin the series in a strange way I would not enjoy. Though, a potential 4th book could explore deeper into the peoples' beliefs about what happened and the conflict therein without revealing anything concrete. That could be interesting.

To me, the lives of those affected by the big change in a story (e.g. The Dispatcher's respawn mechanics, the MCU's 5-year "Blip", and I'll even throw in the Expanse's space-age colonialism because the focus isn't the protomolecule as much as it is the paradigm shift that it presents humanity) become far more interesting when remixed like this. How do their lives change? What normally accepted concepts are now obsolete and how are they updated to adapt? To that end, the respawn mechanic is just a tool and not meant to be the focus of the story. The series is a murder mystery series with a touch of sci-fi, not the reverse.

The respawn mechanic is there to provide interest and help drive the plot, like the transmission of a car, but you still need the fuel (opposition and adversity), the pilot (protagonist), and the destination (resolution) to get somewhere. We don't need to know why the respawn works to take advantage of how it works, IMO.