Everyone hit the main points (HFCS, SAD, and sedentary lifestyle). I'm going to add higher consumption of alcohol:

• Macrobreweries and grocery stores making it easier to always have a six pack the fridge
• Transportation infrastructure making it easier to move wine around the country and next to the beer at your local grocery store
• Drinking culture becoming the norm instead of something you practiced in moderation. Think college culture, spring break, mimosas with brunch, etc.

I'm aware that we have always been a nation of drinkers, but the culture has shifted greatly surrounding alcohol in the past few decades.