Negative intrapleural pressure is due to -
The intrapleural pressure becomes more negative at the end of inspiration and less negative at expiration. It ranges between -3 and -6 mmHg. 
The elasticity of lung and rib cage causes it to recoil and thus creating the negative pressure. 
The lymphatic drainage of the pleural fluid also creates a negative pressure.
Uniform distribution of surfactant over alveoli
Negative intraalveolar pressure
Absorption by lymphatics
Presence of cartilage in the upper airway