A simple psychological term could sum it all up. Response effort is the single most significant factor preventing people from eating healthily. Healthy food is more expensive, therefore it's easier to buy unhealthy food. Healthy food takes more time to prepare, so cooking unhealthy foods saves time. We eat for comfort, and thus we can avoid those more uncomfortable emotions. Simply put, the short-term affects reward us more than working toward the long-term effects.
Most of the factors that prevent people from choosing a balanced diet are not scientific factors. Instead, they are social factors. In other words, it is a person's economic situation or their culture that is likely to prevent them from eating a balanced diet.
For example, some cultures have a traditional diet that is not very balanced. It may be heavy in cheeses and meats like Mexican food is. A person who grows up in such a culture may naturally choose those kinds of food instead of a balanced diet.
Social scientists have also noted that poverty tends to lead to a bad diet. Poor people have a much more difficult time affording things like fresh fruits and vegetables. Their poverty, then, prevents them from choosing a balanced diet.