The reason is simple, because the Pan Peach Banquet offers the Heaven and Earth Spiritual Medicine known as Peaches that vary from three thousand to nine thousand years old. It's known that consuming one of these Peaches can boost an Immortal by one Great Realm, which is why they are extremely coveted by the Group of Immortals.
Of course, not every Immortal can attend the Pan Peach Banquet, nor can just anyone taste the Peaches which are a kind of Heaven and Earth Spiritual Medicine.
First of all, one needs an invitation from the Jade Emperor and the Queen Mother of the Heavenly Court—like the Four Sea Dragon Kings and the Group of Immortals in the Heavenly Court, as well as some of the lesser Immortals who have contributed to the Heavenly Court.
Second, various powerful figures such as the Bodhisattva Buddhas from the Western Lingshan, or the Ten Palaces Yama from the Underworld, and many Daluo Golden Immortals.
