That's easy: increased processed food/sugar in the diet, the official dietary guidelines of high carb/low fat, and an increasingly sedentary lifestyle.
But fat intake in people's diet has been growing every year for the past 50yrs so regardless of these guidelines our fat intake has continued to rise.
I agree that processed foods can help cause obesity because they are so calorifically dense but heart disease is on the rise year after year because of saturated fat intake, which you would know if you followed "silly" studies like the European Prospective Investigation into Cancer and Nutrition which followed over 500k people over many years or the Harvards nurses study which contained 122,000 participants. Both of these studies concluded that higher saturated fat intake was associated with higher all round mortality and the more plant based you got the healthier you are. These are the two biggest studies ever conducted into nutrition, what do you possibly have that counter these?