After the Adal-Abyssinian war both groups had to deal with the Oromo for the next century. This new enemy did not engage armies in a conventional way the way Adal and Abyssinia traditionally met one another in pitched battles, which would justify armour, muskets and cannons.
Instead the Oromo tactic was to ambush their foe when they least expected it, take the Battle of Hazalo as an example, Nur Ibn Mujahid had just killed Emperor Gelawdewos and destroyed his army at Fatagar while suffering losses himself. Nurβs soldiers were injured, and tired, only to be ambushed by a huge Oromo army on what should have been an ordinary journey home. In such a scenario, you have no time to ready your cannons, muskets or even organise your men clad in armour.
Military hardware that once provided an edge to Adal and allowed Abyssinia to resist, was rendered obsolete by an asymmetric opponent like the Oromo.
Another deceptive tactic by them was to draw in pursuing conventional armies into their barren lands, which could not feed the invading soldiers. Once the increasingly famished soldiers were tired of being denied a pitched battle for weeks, they started to make the journey back to their kingdoms, only to be ambushed once again by the Oromo, who were fully fed on their wealth of cattle and energised by the thought of victory.
This is one of the reasons why the Oromo were so successful against the urban populations of Adal and the sedentary populations of Abyssinia but ran into a brick wall when they faced pastoralists in other parts of the Somali peninsula, which is why they could not expand into these lands and why Greater Somalia is so vast. The moment the Oromo settled down amongst sedentary populations is also when Abyssinian and Somali armies had a greater success at subduing and incorporating them.
There is also in this same time period a significant boom in the construction of walled fortifications and ramparts throughout the Somali peninsula, as that proved a much better defence and deterrent against Oromo encroachment than conventional armies clad in armour could ever do. My theory is that this particular change in warfare in the Horn is why chainmail and helmets were abandoned in favour of walled cities, forts and rifles.
