Question: Why is Buddhahood manifested in the human realm?
Answer: Buddhahood is always manifested in the human realm, be it terrestrial (planetary) or extraterrestrial (extraplanetary), as human beings are more or less in the ‘centre’ of the six realms, who will most greatly and fully benefit from learning about the path to Buddhahood. All Buddhas’ teaching influence thus begin in the human realm, from there spreading upwards to the higher realms and downwards to the lower realms.
With the Four Noble Truths being the core structure of all Buddhas’ teachings, it is difficult for Buddhas to teach them completely in the higher realms, where most heavenly beings have too much (but still impermanent) joy. As they are not aware or less aware of (eventual) suffering and its causes — this makes it challenging to understand the First Noble Truth (of suffering) and the Second Noble Truth (of the causes of suffering).
It is also difficult to teach the Four Noble Truths completely in the lower realms, where most hell-beings, hungry ghosts and animals have too much (but still impermanent) suffering. As they are not aware or less aware of the (possible) cessation of suffering and it causes — this makes it challenging to understand the Third Noble Truth (of the cessation of suffering) and the Fourth Noble Truth (of the path to cessation of suffering).
The average human being however, can easily relate to all Four Noble Truths as the human realm is neither too high nor too low, to be too complacent or too despondent respectively. In this human realm, with ample experiences of worldly suffering and joy alternating, thus knowing their impermanent nature, it is easier to seek the transcendental path. The Buddhas do manifest in higher and lower realms too, where possible, to teach beings with affinity. However, initial manifestation of Buddhahood is in the human realm, to firmly lay the foundations of the Dharma.
Although this human realm is a good place to begin learning and practice of the Dharma, it is not the ideal place to progress towards realisation of Buddhahood. This is so as in the absence of the Buddha now, the Dharma can be missed or misunderstood. It is also possible to be distracted by too much worldly pleasures or too much worldly pain here. Life is also shortening in this Dharma-Ending Age with increasing of suffering. Thus, the Buddhas urge all beings, including us human beings, to reach Pure Lands, where Buddhas can be blissfully learnt from directly with immeasurable life, where there are no distractions from the Dharma, where there is the pure Saṃghā, with which progress towards Buddhahood is certain.