X-ray machines have to be calibrated. While you could turn up the power to see through metal, you would then be unable to see the less dense items. This makes it difficult to do a full inspection in the few seconds that the screener has.
There's beyond HDR, you can switch between multiple penetration levels and do automatic detection of materials even with ones made 2 decades ago. None of this is really new, insightful, or particularly revolutionary.
