Answer:

The war on tobacco. JR Reynolds and other major tobacco companies were facing enormous pressure at the end of the 80s when it became abundantly clear they were knowingly increasing the amount of nicotine in their products, making them more addictive, and intentionally trying to hook kids early for increased profits. With lawyers and politicians bearing down wanting to ban smoking and wanting increasingly large settlements from claims they caused people's lung cancers, they made a business decision. They diversified.

They bought into the food industry. And they brought that same mentality to food. It was around this time that the salt content, the added sugar content, and the portion sizes started to increase. The sugar was the biggest component of it. Why? Because like nicotine, sugar triggers a large dopamine release in the brain's reward centers. It's how Nature evolved to let us know where to get calories. You get calories from sweet things like fruits, you little human monkey, you! Well, by the late 80s, we had some pretty good brain scans that showed this effect quite clearly. So, put more sugar into food. It'll taste better and people will eat more of it.

And it wasn't just "junk food" either. They added sugar and salt to everything to make it taste better and boost sales. It worked, and any company that wasn't doing it hired chefs to quality check their own products. They figured out really fast what was going on and replicated it. Within just a few years, this concept had penetrated the market globally. Another poster mentioned high-fructose corn syrup. That's a big part of it too, since it was significantly cheaper than real sugar (sucrose).

And it's not just in the US. Anywhere in the world where there is large-scale uptake of products from the major US processed food companies, you will see obesity on the rise.