I'm wondering from a historical perspective what attributed to this. Like what decade rang in the era of bad dietary habits and why? Can it mainly be attributed to junk food marketing? Is it a cultural thing that caused it? Etc etc
Are there any books on this too?
America is not the most obese country in the world, as can be seen here. That list itself offers some clues, as well as more questions; all the top 10 countries on the list are Pacific island nations, but whether that is genetics or diet I don't know.
You may find this previous answer from u/Superplaner as to the science behind the rise in obesity informative, and hopefully another user can answer your question directly.