Galarian Slowking first rose up to ou during Pheromosa's reign. With an assault vest set, it was the ultimate counter to quiver dance Pheromosa, something that not even the special wall to end all special walls, Blissey, was able to do. After cockroach was banned, it was still being used because of how potent of an offensive Pokemon it is despite being usually used for defensive purposes. A potential sludge bomb poison annoys anything that isn't a steel type and even common resists like Garchomp and Landorus Therian hate getting poisoned. It also can learn flamethrower which scorches any steels, psychic is just a good all around offensive type and even has both earthquake and scald / surf for Heatran
Regular Slowking on the other hand, is in uu because Slowbro is usually better. Their galarian forms do not follow this logic as Slowbro is meant to be a pseudo Landorus Therian that checks the ground types. Minus the ground weakness, poison is actually a very good defensive typing with the ability to resist the dreaded fairies. Galarian Slowking is not meant to be used as a Pokemon that deals with physical attacks. It is supposed to provide you with a backbone against all the powerful special hits while posing an offensive threat itself
Bottomline is, Galarian Slowking is in ou because it is a offensive backbone against special attackers. Most, if not, all special attackers do not have an easy time trying to take it down while regular Slowking is uu because its typing in water and psychic is much better on the physical side with all the you turns and ground types running around, not to mention being able to counter the random Blazikens or Galarian Zapdoses that pop up from time to time
In the current smogon tournament, regular Slowking actually saw a lot of usage, last time I checked anyway, and this can be attributed to powerful special attackers like nasty plot Tornadus Therian, Heatran and Latios around
