Why Doesn't the Buddha Go to the Formless Realm to Teach the Dharma?

The devas of the formless realm have no bodies, yet they have consciousness; having consciousness, they have the aggregate of consciousness. Although they have consciousness, they are in meditative concentration and cannot hear the Buddha's teachings, let alone contemplate the Dharma. Having no bodies, they are not humans among humans, yet they are beings of the heavenly realm—higher forms of life. Feeling, perception, and volitional formations—the five universal mental factors—are present in all eight consciousnesses. The devas of the formless realm also have the feeling, perception, and volitional formations of the mind consciousness, as well as the feeling, perception, and volitional formations of the manas, and also the feeling, perception, and volitional formations of the eighth consciousness.

The Buddha could teach the Dharma to the devas in the formless realm, but the devas of the form realm could not hear it, humans in the human world could not hear it, and ghosts and spirits could not hear it. The devas of the formless realm are so few, and they are all non-Buddhists who enter meditative concentration all day long, so there is simply no need to teach them the Dharma. On the other hand, one can no longer hear the Buddha's teachings even in the concentration of the second dhyāna, let alone in dhyāna concentrations above the fourth dhyāna, where it is even more impossible to hear the Dharma and contemplate it. Therefore, the Buddha does not go to the formless realm to teach the Dharma. The Buddha taught the Huayan Sutra (Avataṃsaka Sūtra) and the Kṣitigarbha Sūtra in the desire realm heaven and the form realm heaven, and then they were transmitted to the human world. All other sūtras were taught in the human world, so that devas could come down to hear the Dharma, and ghosts, spirits, and animals could also hear the Dharma. In this way, many more sentient beings could be delivered.