Why are emphysema patients more prone to infections such as bronchitis?

Poor defense. Emphysema can impair the lung's normal defense mechanisms of clearing bacteria and allow an infection to get established. Chronic bronchitis is actually more prone to develop infections than pure emphysema. Both are under the broader category of copd. Active cigarette smoking also increases the risk.
COPD. Emphysema patients has chronic obstructive lung disease so any infection can set in easily. It is manifested by shortness of breath and coughing episodes. Meds are necessary.