Why is Code Igniter not recommended??

When it says "lightweight" framework, it is more about what it automatically does for you and what it doesn't.

Seriously people, this is PHP, you can do anything with about any framework depending on how much PHP code you want to write.
In the case of CodeIgniter, it does *some* things for you while (some) other frameworks do *everything* for you.

The difference is really boiled down to what level of commitment do you want to have to the tool you're using. To maximize the efficiency of each framework, you utilize and leverage all of the tools they provide you. Ergo, say Laravel, you use Eloquent, Blades, their routing, authentication, CLI stuff and so on. You do everything the Laravel way or the cost/benefit of using it decreases as you depart from the "Laravel" way. For CodeIgniter, you should use the MVC setup with routing, you should use the active class for databases and model the database correctly and so forth to leverage CodeIgniter efficiently. Laravel has more tools that come with it (and, yes, you do not HAVE to use them, but... why wouldn't you?), while CodeIgniter has a complete set of tools sufficient to code out anything with optimal choices on libraries left up to the project/team.

Either platform can accomplish the same things, it really is a difference between the suite of additional tools the frameworks come with and the subsequent dependencies that come with the inclusion of said tools.

The reason "lightweight" refers to the included tools and not the capacity of CodeIngiter is that I've used CodeIgniter to support a SaaS company that supports 1700 clients, at peak times, 50,000 concurrent users at peak times are using the same code base (the database was routed to server farms, but the app was a single instance with each client having their own style/menu/etc differences). The system tied in to dozens of remote APIs for services, managed CC transactions over the internet, dealt with mailing campaigns, accounting reporting, employee tracking (scheduling) and so forth. There wasn't anything we needed the framework to do that it couldn't handle.