Why is Code Igniter not recommended?

I've been developing sites in CodeIgniter since v2.x, and you're right that it's very quick to deploy and easy to work with. It's reliable and performant and makes my servers happy, so it definitely gets my recommendation.

The framework changed a little in v3.x, but much of it has remained the same for a decade+ for a number of reasons. Just my $0.02 ... I think that caused some developers to look at Laravel and Symfony and FatFree as alternatives. Those seemed for several years to have more active development communities and the backing of some larger open source projects (Drupal 8 is built on Symfony, for example).

While CI was largely unchanged, those other frameworks added features like autoloaders, template engines, SASS/LESS compiling, database migrations, debug tracing, etc.

It's good to see that many of these features are being addressed in CodeIgniter v4.x. CI4 is going to work a bit differently than older versions and us developers are going to have to learn/unlearn/relearn how some things are done, but it should put CodeIgniter on a very solid footing for the future.