Here's why this is likely:
* Focusing on the main characters: The books center on the special bond between Annie and her dog Snowball, and the everyday adventures they experience. The author likely chose not to dwell on the absence of Annie's mother to keep the focus on their relationship.
* Open to interpretation: Rylant might have intentionally left it open to interpretation. It could be that Annie's mother is deceased, divorced, or simply not living in the same house. This allows readers to connect with the story in their own way and potentially fill in the blanks based on their own experiences.
* Reflecting childhood innocence: The series is written from a child's perspective, and it's common for children to not fully grasp complex situations like parental absence. By not explicitly mentioning what happened, the books maintain a sense of innocence and focus on the simpler joys of childhood.
Though we never learn the exact details, it's clear that Annie is loved and cared for, and that Snowball plays a significant role in her life, offering companionship and emotional support.