The FAR does not provide an explanation why something is marked RESERVED.  These parts and part 41 were not likely reserved for something to come but may have been originally used/identified  and subsequently been rescinded/superseded/overcome by events.  The prevailing logic is that once something is considered no longer necessary, it is numeric location, i.e., part/subpart/section/subsection/paragraph be marked RESERVED rather than removed.  That way the remaining text that follows does not need to be renumbered.