Why are pregnant women more susceptible to heartburn?

Pressure . There is ralaxation of muscles in pregnancy and the one responsible for preventing reflux would be affected as well as there is pressure from the baby on the stomach causing reflux.
Heartburn & pregnant. Heartburn is expected as part of pregnancy due to (1) upward pressure from the growing baby upon the stomach, and (2) the effect of progesterone in relaxing the sphincter between the esophagus and stomach. There may be some medications that are safe to take in pregnancy, but please consult with your Obstetrician first as not all medications are safe for the baby.
Progesterone. This hormone tends to go up very high quite early in pregnancy. It slows down your GI tract motility...Slows things down. That means food sits in your stomach longer. Plus the added pressure on the stomach causes food to regurgitate into the esophagus. Eat small meals, avoid onions, tomatoes and garlic.