The top reason I like using CodeIgniter is that I can dig into the system files and easily understand how everything is put together. I personally don't like my frameworks stuffed with everything but the kitchen sink with lots of magic methods that make things really convenient until you hit an edge case or you're hitting a performance wall and you have to figure out how to pull it apart. CI has everything a web application could need. For anything else, there's Composer.
The CI codebase is also lean which is another reason why it's easy to understand and follow while also allowing for the code to execute quickly. For reference, one of my full-featured, data-heavy CodeIgniter projects is currently supporting 2 million users and 200 million monthly page views (within the top 5000 on Alexa) with a sub-50ms server response time. CI is a more than capable framework in terms of speed, features, and reliability. I personally hope that CI 4 maintains that leanness. If it follows the crowd, then what would be the difference in using one over the other?
