Guanyin, like her earlier manifestations Avalokitesvara especially is known as a bodhisattva that exemplifies compassion.
That means that Avalokitesvara Guanyin cares deeply about helping all sentient beings, including humans, to attain enlightenment.
For that reason, even this bodhisattva is already ready to become a Buddha, he/she has vowed to delay that final transition until all beings have first been brought to enlightenment.