The Venerable Master Hsuan Hua explains:
“After he had attained Buddhahood, Skakyamuni Buddha spoke the Dharma for forty-nine years in over three hundred assemblies, yet all along, never once did he get a chance to take his mother across [to the other shore of nirvana]. Shakyamuni Buddha was born from his mother’s left ribcage, and his mother passed away after giving birth.
When he became a Buddha, he learned that his mother, Lady Maya, had ascended to the heavens. After he had spoken the Dharma Flower Sutra and before starting the Nirvana Sutra, he thought of his mother and ascended to the Palace of Trayastrimsha Heaven. He stayed there for three months to expound the Dharma for his mother. And what was that Dharma? It was the Earth Store Sutra, the Sutra on filial piety.
For the sake of crossing over his dear mother, Shakyamuni Buddha spoke the Sutra of the Past Vows of Earth Store Bodhisattva in the Palace of Trayastrimsha Heaven—those are the causal conditions leading up to this Sutra.”