People who smoke or have lung conditions often have less capacity to absorb oxygen. They also tend to have some lung inflammation. Normally it isn't a big deal, except for people with really bad COPD or asthma. However, since coronavirus can cause inflammation, trouble breathing, and shortness of breath too, smokers and people with lung conditions struggle more when coronavirus is impacting their lungs.
Read more about COVID-19 and lung disease at the American Lung Association