Still others may wonder why the Buddha could not liberate all sentient beings with the same skillful means. The sutras said that for beings to encounter ‘liberation upon wearing’ or Bardo Thotrol (Liberation through Hearing in the Bardo), they must have had certain causes and conditions occurred in their prior lives. What does it mean by past causes and conditions? For example, one can learn Vajrayana, the Great Perfection, or listen to profound teachings such as the Tantra, Single Heir of the Doctrine in this life, but may still take rebirth in the lower realms due to improper practice or broken vows. In that case, after being in the lower realms for a very long time, one may ultimately be saved not by the exoteric practices but that of the supreme Vajrayana rather effortlessly. This is because one has previously planted the good seeds of being exposed to the Vajrayana teachings and thus accumulated the merit that eventually allows one to be liberated by the inexplicable power of hearing or wearing liberation.
[Excerpt from Luminous Wisdom Book Series 1: The Right View ~ Liberating Living Beings]