Moff Gideon led a sizeable faction of the remaining Imperial military in 9 ABY (five years after the Battle of Endor), which included a platoon of forty Phase III Dark Troopers. With elite droid units as deadly as the Imperial third-generation Dark Troopers, it’s curious why a successor unit wasn’t used by the First Order, but there are several reasons why this might be the case. The First Order’s TIE Fighters were equipped with shields, their Stormtroopers were trained since infancy, and their superweapons outclassed the Death Stars in destructive power.
While First Order leaders often lack the discipline and competence of their Imperial predecessors, their technology is almost always superior. The Mandalorian bridges the Star Wars original and sequel trilogies by showing the Imperial remnant factions as they transition to the First Order, but why doesn’t Kylo Ren’s army use Dark Troopers? The First Order sought to improve on the Empire in every way, and in terms of their weapons and military, they were largely successful.