That's because it hasn't been designed/tested/sold yet.

The article is about TSA asking tech companies to come up with something (they don't really care how it works) that will scan shoes. Could be radio-based, could be x-ray, could be detecting trace chemicals.

Or, you know, it might just be impractical to build a machine that scan all kinds of shoes in every possible scenario in a reasonable amount of time with adequate accuracy. But I don't think that will stop TSA pouring more money down this particular hole. (It's certainly a good thing that US economy is so robust and that the US gov't has such a huge surplus to fund this sort of speculative R&D.)