The following are the reasons why codeigniter is one of the famous frameworks:
1. MVC Architecture: MVC is model, view and controller architecture. Almost all frameworks are based upon this. If the large websites or applications are not followed by this pattern then it ends up with unmanageable spaghetti code. MVC offers nice code separations and make things clean. Some frameworks force you to do work from books but CI use MVC that makes a sense.