Why does the female bandicoot's pouch open rearward?

To prevent dirt and debris from fouling the developing young inside while digging for food.

The rearward orientation of the female bandicoot's pouch is a significant evolutionary adaptation directly linked to their fossorial, digging lifestyle. Bandicoots vigorously root and dig in the soil to excavate food items. If the pouch opened toward the front or top, the vigorous activity of searching for sustenance would constantly force soil, dirt, leaves, and debris directly into the pouch, potentially fouling or harming the altricial young developing inside. The backward-facing opening effectively seals the opening away from the immediate source of disturbance during foraging activities, providing a cleaner and safer environment for the undeveloped infants as they complete their growth cycle attached to a teat.