Unhealthy is the only option
There is a simple explanation of why people choose to eat unhealthy food rather that healthy options at fast food restaurants: they taste terrible. In this day and age, to try a salad in McDonalds is unheard of. But if one was to try a salad, i am positive they would never consider having it again.
If the fast-food companies are really interested in giving people healthful alternatives, they should work harder at making them appealing and palatable. For most people who live in the city, fast food is the only option on their lunch breaks and many would appreciate a healthy option that tastes good, but when the choice is between a bland salad and fries, almost everybody would go with the fries.