Lavos was bound to the Darkness Beyond Time (DBT for short) because it only existed on a discarded timeline, the ruined future after the Day of Lavos. Schala was not rewritten out of time; she wound up in the DBT after being drawn into the dimensional distortion surrounding Lavos in the Ocean Palace. She, Serge, and the others physically exist in a real sense, and that is why they are not trapped in the DBT. Lavos is only a shadow of a forgotten timeline, and without Schala's power, it is simply frozen within the DBT.