The other four enhancers have a rarity of uncommon, but kinetic and ion are rare. In addition to causing them to show up less often, this also means they can only be ordered one at a time, instead of three.

In the case of ion, this is probably a relic from when Longzhu spheres could enhance already enhanced energy weapons (and before internal hp made energy weapons significantly worse). I can't think of any justification for kinetic enhancers being so rare.