Why are some people more prone to hernias than others?

Multifactorial. The cause of a hernia may be different among patients, and even on the same patient over time. One herniaaybne caused by an operation in the setting of obesity and complicated by an infection. Another may be caused by a car accident. What is also important, but unknown is the fact that many with hernias have abnormal connective tissue or collagen. Why and who are still mysteries.
Congenital, Trauma. Many hernias develop during fetal life, although they may not manifest themselves until adulthood. Hernias often arise after abdominal surgery, and are more likely to occur following midline incisions. People who had wound infections, are overweight, and/or do a lot of heavy lifting are more prone to these types of ("incisional") hernias.