Always makes me laugh when people forget that Mahoraga's regeneration only ever happens when it starts Adapting to your attacks...
That's why he went with Open at the end of the fight, not because it was "to completely disintegrate Mahoraga", it's to simply hit it with an attack it never adapted to and in the typical Sukuna fashion, it was with absolute overwhelming force.
Gojo, meanwhile, had to deal with multiple enemies and Sukuna locked off Red from him with The Wheel.