I can only speculate, as whoever made the decision I would suspect is long past, unless one of our older members knows.
Aluminum is one of the metals that is especially susceptible to Galling. Brass and Bronze less so and are often utilized for moving parts, such as bushings and bearings, etc. It is possible the materials were chosen specifically because of those characteristics?.
One way of preventing or at least limiting galling is by the use of lubricants, not something you would really want in and around powder.
