Can you please explain why asthma is classified as an obstructive and not restrictive disease?

Obstruction. Airway "obstruction" refers to limitation of air flow through the airways. Asthma is a disease of chronic airway inflammation that can "obstruct" flow. "restriction" refers to processes that keep the lungs from opening fully when you inhale. Restriction can result from fibrosis, or thickening, of the lung. It can even be from something as simple as obesity that keeps you from breathing deeply.