Why is the intrapleural pressure negative rather than positive?

Answer 1

To maximize total lung volume. The parietal pleura has to be as close as possible to the visceral pleura. There is a thin layer of fluid between them that acts as a lubricant. Negative intrapleural pressure is apparent most when you are breathing in.

Answer 2

Inhalation is usually the active process in the respiratory breathing cycle. When the chest expands it causes an increase in volume of the chest cavity. The increase in volume causes a decrease in intrapleural pressure and the lungs expand, filling with air.

For the physics of why, use Boyles Law: P1V1=P2V2, where volume and pressure are inversely related to one another. An increase in chest volume causes a corresponding decrease in the pressure of the interpleural space. The opposite occurs in exhalation.