Due to its stringent size tolerance, hexagon socket head screw is very likely to cause awkward situations where the hexagon wrench can't go into to socket head or strips during the insertion. M2.6 requires very high precision. Particularly when it comes to electroplating, the width of coat on the hexagon socket is very likely to cause the problems mentioned above, so makers usually don't want to produce such screws.
Due to that problem, makers usually choose to make such screws in stainless steel. However, Nagai Seimitsu Company specializes in small diameter hollow head screws and supplies electroplated screws free of those problems. Moreover, last year it rolled out trivalent chromium plated and copper-based nickel plated screws ranging from M1.6 to M5.0.