Why do some people get adhesions and some don't?

Unknown, but... Genetics plays some role. Any surgical procedure of the pelvis or abdomen can cause adhesions; more so if inflammatory/infected surgical field, e.g. Pus, stool, blood. Less likely with careful handling of tissue, and elective rather than emergency procedures. Most everyone gets adhesions; a minority get aftereffects (pain, bowel obstruction) even decades later.
Abdominal adhesions. Adhesions are a type of scar, and a part of the healing process so present in everyone. The issue is symptoms related to the adhesion. Some people develop bowel obstruction when loops of bowel are adhesed together, occasionally requiring surgical intervention. It is not clear why some people develop bowel obstruction related to adhesion, while most do not.