Because he was the only one who had achieved Foundation Establishment with the Spirit Nurturing Technique.
Take Yuee Mengsheng, for example. He had a Talent for being a Spirit Nurturing Master, and his Spirit Root was decent enough.
So, he had always been aiming to become a Spirit Nurturing Master. For his Tai Chi Foundation Establishment, he chose the Tier Two Ice and Snow Heart Nurturing Skill as his Rune Core.
His subsequent Three Lives Rune and Duality Cultivation were all based on similar techniques.
Step by step, he paved the way to learn the Spirit Nurturing Technique, as all these skills were intricately related to it.
Although they weren't direct developmental techniques, they were still connected. Thus, as he accumulated them bit by bit, the difficulty of learning the Spirit Nurturing Technique slowly decreased, effectively lowering the entry barrier for him.
