I was reading about gzip program and I found the official website of the gzip software.
No, you didn't. You found the website that used to be the official website back when the original authors were still the maintainers. Now, GNU Gzip is maintained by different people, and the website is https://www.gnu.org/software/gzip/ .
First of all the site looks very bad,
That is in the eye of the beholder. I find it simple, with no extraneous fluff, it loads extremely fast, and it works well in a mobile browser on a small screen.
Sure, the markup is horrible, but the authors are C programmers, not web designers.
it is very hard to find useful information on it
That is not my experience. Also, define "useful"
and it was not updated since several years ago.
Yes, the people who own that website no longer maintain Gzip, why would they update it?
Also, only www.gzip.org is working, but simply gzip.org is not working.
Those are two completely different hostnames. There is no reason for them to work the same way or work at all.
It would not be hard for people that are running those websites to make it better so that it would be more easy for more people to have acces to open source software and learn about it.
What is the reason no one cares about User Interfaces and accesibility for those software/websites ?
What makes you say no one cares? Obviously, you care and you are not no one. It's an open source project. The work is done by the community. That is you.
On the website the latest update is about a patch for version 1.2.4, but on the wikipedia page it says that the last STABLE release is 1.8.
That's because you are looking at the website for the project that no longer maintains Gzip. If you look at the current website, you'll find Gzip 1.8 right here: https://ftp.gnu.org/gnu/gzip/
What is the reason for these inconsistencies? Was the project abandoned by the creator and the owner of the website
Gzip was originally maintained by Jean-Loup Gailly and Mark Adler, who also maintained the http://www.gzip.org/ website, and is now maintained by Jim Meyering and Paul Eggert as part of GNU, which maintains the https://www.gnu.org/software/gzip/ website.
and is now only developed through platforms like GitHUB?
No, most GNU projects are managed on Savannah, not GitHub. Here's Gzip: https://savannah.gnu.org/projects/gzip/
Even so there has to be somebody to take care of the development process.
Yes, there is. Jim Meyering and Paul Eggert and the GNU project.
Wouldn't it be a good idea for the free software community should take more care about popularizing their projects ?
Gzip is installed on more machines than Windows, OSX, Linux, Android, and iOS combined. It runs in space, and the depth of the oceans. It runs on watches and supercomputing clusters. How much more popular could it be?