Answer
The location Kūkai selected for Mount Kōya was chosen because it was considered a holy site. According to Shingon tradition, this area was sacred to the deity Fudō Myō-ō, who appeared to Kūkai in a vision, urging him to establish a monastery there. The mountain's remote, secluded nature was believed to be conducive to spiritual practice, free from worldly distractions and conducive to deep spiritual contemplation.