Why does the hook grip work so well?
When taking a normal, double-overhand grip (i.e., without the hook) on the bar, you’ll notice that you have four fingers on one side of the bar and only one finger (your thumb) on the other side of the bar. It is the thumb that fails you when the load gets heavy - it leads a lonely existence on that side of the bar, and it doesn’t have any friends to help.
The hook grip remedies this situation by allowing the other fingers (primarily the middle finger) to help the thumb maintain its purchase on the bar. I suspect that the thumb also starts to fail in the non-hooked grip due to the angle of attack it has on the bar - note its somewhat “from-the-side” approach to gripping the bar compared to the other fingers’ perpendicular approach to the bar - but I haven’t given this enough thought just yet. Nonetheless, suspicions abound.