The fast food industry is a very competitive industry. The different restaurants compete on price, quality, service, menu, as well as other factors. Overall, all of the different chains have similar prices, on a per person basis. It is very typical that when one restaurant has a special that a competing restaurant will have a similar promotion.
Different chains cater to different customers.
Wendy's , for example, caters more towards a grown up customer. McDonalds often caters to families with younger children by promoting their Happy Meals. The industry has grown with the addition of new technologies. The Drive Thru window, revolutionized the industry. People could now purchase a meal without even having to leave their car.