But what would it have to be about, for employers to consistently turn down an approach that gets them access to a bigger class of labor and better filtering for safety?
Good question, and worth investigation. While you do, please keep in mind that employers are not monolithic, but made up of lots of different humans with their own agendas. (And lots of those agendas include the objective of "cover your ass".)
There's a great book about (what might be) the general problem here: that there are many factors that keep us at a bad equilibrium, where a bunch of people would have to change at once to accomplish anything.
Here, I think it's something like, "insurers move very slowly to new systems, and until this new testing has a lot of data behind it, they won't adjust premium. to account for it being better. Furthermore, juries will continue to give punishingly big damage awards for using it, on the mistaken believe that a toker shouldn't be on the job even if he wasn't high and passed the reaction test."
