Solution:
odeigniter is a development framework for building applications. WordPress is a system that’s already built.
I think you could build a large application like WordPress with Codeigniter. But it would then rely on Codeigniter’s libraries and framework, so to make that happen I think you would have to rebuild WordPress from scratch. Unless Codeigniter can be used as more of a PHP editing framework as well?
Maybe a Codeigniter guru will weigh in.
Source https://wordpress.org/support/topic/codeigniter-wordpress