In addition to the requirement of a more active approach needed to detect ground/water contaminants there are also a larger variety of pollutants that are harmful. Many of these need specific tests, which can consume your samples. Of course we can do pretty good guesstimates for what we should look for, but we do need to recognize that the process is both more fuzzy and more involved. We can grow these projects by making them cheaper, but that's a tall order (it is happening though).
Edit: I do want to note that most radiation detection devices do not distinguish between types of radiation. These differences do matter in danger levels. This can add complications the above but there is a decent signal that is still useful. But as with everything, some expertise and domain knowledge is quite important.
Anything like that radioactive source from Australia would set of tons of alarms in a nuclear power plant. You wouldn't get it out the door. Incidents like that missing radioactive source in Australia happen where there are far fewer safeguards than at a nuclear power plant. Those sources generally go missing from abandoned medical equipment, food irradiation facilities, and that sort of thing. You'd be hard pressed to smuggle (let alone accidentally convey) something like a spent nuclear fuel pellet out of a power plant.