Why do patients with chronic bronchitis develop hypercapnia?

Most likely because: the cause of the chronic bronchitis is obstructive lung disease, otherwise known as COPD. When the CO2 is trapped because of bronchial constriction (narrowing of the breathing tubes) and poor alveolar diffusion, it cannot be exchanged with oxygen and the CO2 remains in the blood. When that happens, the pH of blood increases and the patient suffers from high degree of acidity in the blood.

COPD: Patients with chronic bronchitis together with COPD, chronic obstructive pulmonarry disease usually have hypercapnea.Major complications of chronic bronchitis are severe shortness of breath, COPD, respiratory failure, and an increased mortality rate.

Emphysema: Chronic bronchitis is associated with emphysema which reduces gas exchange in the lungs leading to carbon dioxide retention and lower oxygen level. I trust you are under the care of a pulmonologist.