I've watched a video about this recently. The problem is, most detectors were based on X-rays in the past decades. Liquid explosives are pretty close to the density (and/or other properties) of water, and you can't tell for sure whether there's toothpaste or boom juice in that tube.
However, some airports started using expensive MRI MRI like X-ray* machines that can see stuff in more detail, plus, it lets you to make cross sections from different angles and therefore have a 3D model that you can rotate on your screen (it's rather cool).
EDIT: I just realised someone else linked this, too. I would leave it here, it's still educational.