Some measurement mics still commonly use stainless, nickel or titanium diaphragms. I suspect one reason is to help ensure improved thermal stability by constraining capsule construction materials to those which have all have very similar thermal expansion properties, so that the entire capsule assembly to shrinks or expands in an isotropic way without varying diaphragm tension as much as might occur otherwise. I suspect use in measurement mics may also have to do with retaining calibration over longer time frames, and/or application in environmental conditions which require such materials.
But don't overlook the influence of simple historical inertial. Designs are tied to the materials and manufacturing techniques available at the time. Once developed, sufficient economic incentive is necessary before it makes sense to redesign for new materials or manufacturing processes. A new design uses currently available tech, whereas older designs retain the use of what was then contemporary but is now old tech, and economic factors determine whether those older designs remain in production as originally designed, are discontinued, or end up being redesigned.
Also keep in mind that stainless steels are alloys containing various amounts of chromium and nickel. Metallurgically, there may not be as much difference between a "nickle" and a "stainless" diaphragm as one may imagine.