It's probably not one factor but many. I'm not entirely convinced that HFCS is any different than sugar, and sugar hasn't really gone up in the lasty 20 years. Seed oils and grain "products" have. What I think might be behind a large chunk of it is that epigenetic programming in utero. Babies born to overweight people and consumed bad food during pregnancy will be at a higher risk for metabolic syndrome, regardless of the type of environment they are raised in. The problem will compound with every subsequent generation unless people get a really good handle on their problem prior to procreating.
Stabbing conventional wisdom in its face.
Anyone who wants to talk nutrition should PM me!