Why do switches and relays have lower current ratings at higher voltages?
I would imagine that the current derating at higher voltages would be due to arcing when the contacts open. Keeping the same current as voltages increase will allow arcs to persist longer and cause more damage to the contact surfaces. On small relays and contactors, these arcs are tiny, but if viewed in a darkened room, you can see that they do exist. Over several thousand cycles, (especially with inductive loads such as motors) the arcing will cause pitting and oxidation of the contact surfaces. Damaged surfaces are more resistive, which heats the contacts, and promotes more arcs. Failure will come much sooner under these conditions of accelerated wear.