Why do some people in 20s & 30s develop heart disease? is it from eating bad? or mainly because of family history?

Both, more family: Developing heart disease from blockage of coronary arteries is, fortunately, uncommon in the 20's/30's and if due to poor diet, many more people would have the problem! Most often there is a family history with genetically abnormal blood fat/lipid problems or high blood pressure. Early and lots of smoking also contributes. Very important for those with family history to optimize risk factors:)