Exactly what is meant by the empty space in the lower part of the sahā world below the earth is unclear. Probably it is simply the most convenient way to have this huge number of bodhisattvas be hidden, yet not be in the less than human regions within the earth, and not be among the heavenly deities, yet still be in the sahā world. In other words, both for the sake of the story and for the sake of the central message of the Lotus Sutra, it is important that these bodhisattvas be both hidden and of this world.
A Buddhist Kaleidoscope; Gene Reeves, The Lotus Sutra as Radically World-affirming, Page 184