I'll talk mainly about the Ottomans

Well of course in early war it's obvious, as everyone else is wearing leather/cloth caps (Pickelhaube, British Cap, Kepi, etc). Later on, when steel helmets become mainstream, the Ottomans couldn't adopt it immediately either, mainly because military production deficiencies in the Ottoman Empire. The military industry mainly centered in Istanbul, and could only produce a meagre amount of equipment. The Zeytinburnu factory, which used to produce steel equipments such as bayonets isn't good enough to produce a lot of steel helmets for the Ottoman army. So the only ones who wore steel helmets like the Yıldırım army group and Ottoman expeditionary forces to Galicia got a slightly modified version of German M16 helmet.