Performance standards are used in several ways: to gain FDA approval, to make marketing claims, and to test assays after release for sale that are in routine use.
Using glucose meters as an example…
Endocrinologists, who care for people with diabetes, would be highly suited to writing standards. They are in a position to know the magnitude of error that will cause an incorrect treatment decision.
FDA would also be suited with statisticians, biochemists, and physicians.
Companies through their regulatory affairs people know their systems better than anyone, although one can argue that their main goal is to create a standard that is as least burdensome as possible.
So in the case of glucose meters, at least for the 2003 ISO 15197 standard, regulatory affairs people ran the show.