I’m pretty sure it’s primarily changing diets that include way more carbohydrates, combined with lifestyle changes like increasing use of automobiles. I have Type 1 (the far less common autoimmune version, which is really about the opposite of type 2 with some overlapping effects) and the amount of carbs in typical American meals blows me away. Type 2 is essentially insulin tolerance (called resistance), like one gets to psychoactive drugs with repeated intake. People eat such an insane amount of carbs (not just sugar, but rice, potatoes, wheat, corn, tapioca) that their bodies have to release a flood of insulin to try to maintain glucose levels… and after a while it just stops working as well. The thing is Type 2 can be managed pretty well simply with low carb or keto diets but most people aren’t willing to make that change to their food intake.