While it's certainly true that the drugs and driving committee does not recommend to the government that sweat should be included at this stage, the technology is basically in its infancy. Its utility is limited. We do have blood, which is the gold standard. This is the one that's active in your system right now affecting your brain, and would be the one that would be preferable in most circumstances. Urine is very easy to collect—it does not require a specialized thing—and, of course, they can take an oral fluid sample. Oral fluid is recent consumption. Sweat is not. In that way, sweat is somewhat like urine, not as usable.
The main problem, according to the drugs and driving committee, is reliable collection without having it contaminated. They would have to develop entirely new processes for analyzing this stuff in the laboratories. They are not equipped to do that as they are equipped now to do the other bodily substances.