More than likely there is a section in the codex astartes regarding close combat, including lay out of specific styles of fighting that include chainswords predominately. It may very well be that loyal Astartes concentrate on a more balanced form of combat which would include the use of a more balanced and versitile weapon A.k.a the chainsword as opposed to the chainaxe.About all this wishlisting on chainaxes, I think special rules for the humble Chain Axe feels incredibly unnecessary. If they are so awesome, how come the loyalists stopped using them? If they can create chainswords, chainaxes would be easy. If they are superior to such a degree that they should have special rules in the 40k scale, it would just be silly of the loyalists to stop using them
Anyone who has wielded as sword and axe before knows that they both feel extremely different in your hands. The sword is much more balanced and easy to wield while offering the ability to thrust, cut, and defend, where as the axe's weight is distributed much more to the top of the weapon offering a much harder blow that is more beneficial to penetrating armour. It is also harder to defend with.
The World Eaters are known for their brutal assaults therefore they would require a weapon such as the chainaxe to augment their already formidable assault/combat ability. Astartes in general would probably prefer a sword as a way of counter attacking/defending after volley fire from bolters at close range.