There are a variety of reasons why people don't eat "healthy".
1. Some people never ate healthy during their childhoods and therefore never acquired the taste for eating many of the foods that are considered healthy. Junk food and soda are addictive.
2. Speed, variety, and convenience of fast food is difficult to beat. Historically families had home cooked meals but today many of them have take out food, fast foods, pizza delivery, or frozen foods almost daily. Those who bring lunch to work often have processed lunch meats or soups with high levels of sodium.
3. The cost of healthy food or organic food is significantly more expensive than many fast food offerings. The only way you save money on healthy food is if you cook at home. Aside from the occasional BBQ, Holiday meal, or special celebration most people don't cook meals at home that often anymore.
As of now in the U.S. one can go to Costco and buy a foot long hot dog, (large slice) of pizza, a cup of frozen yogurt, and soda with (free refills) for about $5. On the other hand a small Cesar salad is around $3.99 by itself with nothing to drink and not nearly as filling. Maybe that's why it's not called "comfort food".
The average person with $5 to spend on lunch is probably going to attempt to get the most bang for their buck. Maybe if salads were $2.99 or healthy/organic food in general were priced lower more people would be tempted to eat them more often.