The main reason Cursola is in NU is because of its Horrible Physical Bulk along with Speed. It has 60/50 HP and Defense, along with 30 Speed. This forces it to run a Focus Sash and not run anything to take advantage of its wonderful Special Attack and Defense. Even with Weak Armor, Cursola's Speed still is horrible. Its horrible Physical Bulk led Cursola to NU.
Now, onto Galarian Corsola. This is mostly because it is outclassed by other Eviolite users such as Porygon2 and Chansey. Even though Both have a Higher HP which is desirable on a Wall. Porygon2 Does have slightly lower Defenses, but it overall has more bulk. It is the same with Chansey, but not the Physical Bulk. Now this is just stats, now let's move to roles. Corsola runs a Physical Wall set with Will-o-Wisp and Strength Sap. Even Corsola is outclassed by Physical Walls such as Hippowdon and Skarmory who has access to Hazard Setting and annoyance with Whirlwind. While Corsola may have more bulk combined with its moves, it was still worse than these 2. Also, another reason may be the Common Urshifu- Single and Dragapult capitalize on Corsola's weaknesses. So, It being outclassed by other Eviolite and Physical Walls led to it in NU as well.