Because in Buddhist Cosmology, the Buddha (and other Buddhas) rank far, far above the Devas. The Jade Emperor is Sakra/Indra, the King of the Gods, but even Gods are still caught in the web of Samsara, unlike the Buddhas, who have achieved Nirvana/Enlightenment.
如來佛為什麼可以替玉帝做主？
Because in Buddhist Cosmology, the Buddha (and other Buddhas) rank far, far above the Devas. The Jade Emperor is Sakra/Indra, the King of the Gods, but even Gods are still caught in the web of Samsara, unlike the Buddhas, who have achieved Nirvana/Enlightenment.