How can a framework give you a tuff time to install? It's a php library.
Symfony2 can be a pain to install for not so experienced programmers. Specifically due to file permission issues. They even highlight this in their own documentation. I wouldn't really call it 'just a library', when it requires advanced command line usage.
Just because it's popular doesn't mean it's well designed.
Codeignitor is easy to use, and therefore has a pretty good following of people who are new to frameworks and don't know any better.
From a design perspective, CI is absolute rubish. I know people who were at one point in time making all of there cash through building web sites on top of this platform. Even these guys (who were being flown around the world for work) have moved to other, more modern, better designed frameworks and are once again, making a killing.
You still haven't backed up your claim that CI is poorly designed and 'absolute rubbish' with any examples of why? I don't think you can judge a framework on how much money people make from developing using that framework. Just because something is (or more importantly, is not) trendy, doesn't mean it is the best / worst.
Codeigniter is a great framework for projects where you may have no choice but to use shared hosting, with perhaps a not so up-to-date version of PHP. In that situation, choosing CI could save you a lot of time and stress.
Sometimes I think it is just considered 'cool' to shun simple and effective for overly complex and superflous.