One reason is the use of antibiotics. Data shows the use of antibiotics greatly increase the survival rate of child between birth and 5. Without antibiotics, the survival rate is 30% while with antibiotics the survival rate is up to > 95%. For older generation, each and everyone of them are winners because they passed the selection of Mother Nature, therefore it is no surprising they are more resilient to disease. For most people born after 1960, we are not selected and 70% of us shouldn't be here because we are just not strong and healthy enough in the first place.
Ask yourself, how healthy do you think you are? Are you confident you belong to the 30% genetically?
This is just one reason. Other factors, stress, lack of exercise, over eat, pollutions, are not helping us either. This is pure game of chance, someone will be disease free no matter what, but for most, it is a matter of time: something can go wrong will go wrong.