Many reasons. Smoking coats the lungs with a layer of tar and heavy, chronic smokers have coated their lungs to such an extent where there is drastically reduc

Many reasons. Smoking coats the lungs with a layer of tar and heavy, chronic smokers have coated their lungs to such an extent where there is drastically reduced lung area in contact with the environment (and irritants/allergens). The tar acts like a shield in this case and prevents immune response that can lead to constriction in the lungs and phlegm production. Moreover, active smokers have destroyed the cilia in their lungs (which start to regenerate when one quits smoking) and it's the cilia that do most of the cleaning and phlegm expulsion. When one quits smoking, he goes through weeks or months of coughing and increased phlegm production as the cilia start growing back. Finally, a lot of active (and ex) smokers do cough up phlegm. The co-factors of phlegm production are simply too many for a definite answer. You also have asthmatics and those who suffer from COPD/emphysema.