Several reasons. This question is not fully answered. It seems in patients with emphysema the lungs ability to defend against infection is decreased but the specific areas of impairment are not fully known. They probably include changes in the cells which fight infection or identify infection, changes in the lung produced by chronic inflammation, impaired ability of the lung to clear mucous are a few.
Retained secretions. The dilated bronchi accumulate secretions that in a healthy person are moved up and out of the lungs. These secretions tend to get infected.
Factors... Emphysema patients have chronic airway obstruction which makes it difficult for them to exhale properly which can leave secretions behind in the lung which can then become infected. Also, there is chronic airway inflammation which also increases infection risk. Lastly, they tend to be more synptomatic with even minor infections, making it appear that they have more infections than normal patients.
Damaged bronchi. Emphysema doesn't usually occur as an isolated condition - it's part of a spectrum of chronic lung diseases all characterized by damaged breathing passages. Thus resistance to infection is impaired and excess mucus is often produced resulting in cough, phlegm, and susceptibility to invasion by bacteria.