The second most obvious reason is because fast food is, well, fast. There is very low or no waiting period. Food is usually prepared in matter of minutes and you can be on your way.
Most fast food restaurants also have drive-thru’s, which allow customers to order even more quickly. Now we don’t even have to get out of our cars to purchase food. When compared to traditional restaurants, you could be saving over an hour of time from ordering to finishing your meal.