I realize via looking it up in the dictionary that dropsy is an antiquated term for edema, which is swelling. But as a 21st century person, I've never heard of anyone either dying of edema or being so swollen up that it's a ubiquitous medical problem. Though I realize edema can be a symptom of a larger illness, for example I had it when I had pre-eclampsia during my pregnancy.
Why were people once so commonly swollen up that they needed a special word for it?
Although dropsy would refer in general to swelling, it more specifically refers to congestive heart failure. People did die very frequently of congestive heart failure, and continue to do so. Heart disease continues to be a leading cause of death. Per CDC statistics, heart failure contribute to about 300,000 deaths per year in the U.S. The issue with edema, especially with heart failure, is that it causes fluid buildup in the lungs. This is a frequent cause of death in heart failure. Up until the early 20th century, things like infections and trauma were the leading causes of death. However, heart disease would still have been a frequent cause of death.