This narrative doesn't explain why In 'n Out is very popular. They have five items on their menu. http://www.in-n-out.com/menu.aspx
(They also have a secondary menu, yes, but that's more for stylings and extras.)
I would describe it as McDonald's offering a very wide and shallow menu, while Chipotle/Five Guys/In-n-Out offer a very narrow but deep menu.
McDonald's is experimenting with touchscreen kiosks to let you decide from a list of burger topping options.
This narrative doesn't explain why In 'n Out is very popular. They have five items on their menu. http://www.in-n-out.com/menu.aspx
(They also have a secondary menu, yes, but that's more for stylings and extras.)