Scientists have generally understood that the mechanism of superconductivity in copper oxide compounds depends on the ability of electrons on adjacent copper atoms to pair up.
Each copper atom has a single, unpaired electron in its outermost energy shell, or orbital. Removing some of the electrons that reside on copper atoms results in electron vacancies known as holes. So instead of filling up electron orbitals, electrons in several outer energy orbitals remain unpaired, yet aligned with one another and electronically active.
The alignment of unpaired electrons in multiple orbitals gives simple iron its strong magnetic and metal properties, so it’s easy to see why iron compounds would be good conductors.
Perhaps unpaired electrons in one particular orbital could pair up with electrons in the same orbital on an adjacent atom to carry the supercurrent, while electrons in the other orbitals provide the insulating, magnetic, and metallic properties.
The big difference is that in iron selenide, these contributions come from different electrons in three separate active orbitals, instead of the single electron in one active orbital in copper.