Because if it's sensitive enough to detect a bomb it will be set off by marble, bananas or a dozen other things.
So the port only checks containers likely to have radioactive contamination, like scrap metal, which they did and they found this - system worked precisely as it should.
You could have monitors on the cranes to detect very high levels of radiation, so high you don't even want to check it manual - as in this case, But these happen so rarely it's not worth it. And with 250 Million container movements a year what sort of error rate would you need to achieve?
And, as is mentioned in the article, pretty pointless. A bomb can be shielded to the point of giving off less radiation than normal cargo.
That said, the eventually used a much more precise germanium-based detector that was able to identify the radiation by source. Such a detector could distinguish the decay of shielded U-235 from a shipment of bananas. I wonder how much those detectors cost...
"The team then brought in one of the most sensitive portable detectors on the market, an $80,000 Ortec HPGe Detective DX-100T. Inside the unit, a 1.65-pound chunk of germanium..."
I have no idea if this is a reasonable price, but the mentioned product is a self-contained measurement system in a ruggedized enclosure with built in computer and software that identifies isotopes automatically. It is marketed to "homeland security" type agencies, so expect it to have quite a sales-margin :-)
If you'd want to be on the cheap side, there's a used Ge on eBay for $3k right now, add a HV supply anda suitable ADC card for your PC and you can go recording γ-spectra in no time, possibly on a $5k budget.
Not sure if there are more robust bits of hardware about, but my experience of radiation detectors is that they are very sensitive and have to be very carefully handled. And docks tend to be fairly rough environments.
