Sugar and refined carbohydrates are two big culprits. Then changing the balance of HDL-cholesterol to LDL-cholesterol changes with weight gain making it all worse and possibly leading to a positive feedback loop. The historic denigration of all fats, good and bad, helped to further tilt the HDL to LDL in the population making lots of people less healthy. It isn't HFCS, it is the over use of sugar in most of our food. This is especially true in North America, but then we exported much of the same food tech to the rest of the world who did the same.

All that processed food? Full of refined carbs and sugar. Drinks? Often full of sugar. Cheap food? Usually highly processed and refined, so more sugar and refined carbs. You need carbs for energy and fats to keep everything going with balance of nutrients and protein. Any of that out of balance and health suffers. Too little fat can even kill you (rabbit starvation/protein poisoning). The modern diet in North America is terrible because we were told good things were bad and carbs were good. So we ate too little of the good fats, too much of the refined carbs, and too much sugar. Now were here, increased heat disease, diabetes, obesity, and cancer.