Is age. Bad luck probably comes in second:
This is something that gets obscured in today's medical reporting--everyone, as they get older, will get sick for some reason and die at some point. The biggest risk factor for heart disease and cancer isn't BMI, cholesterol, blood pressure, etc....it's age. And to a large extent, the incidence of those diseases is random; doing everything right lowers your risk of disease, but not to zero, and usually not to that small a fraction of the risk for the general population.