TheSaint I am presuming you have been able to open 7Z files for years in Linux, just not able to create them until p7zip came along?
I think you misunderstood what p7zip is, it is an independent project started to port the tool to other *nix OSes... the development for p7zip has been stagnant for a while and there are known security issues in it, so people generally avoided it.
Now the author of 7-zip has decided to finally add support for Linux & co. in the official 7-zip program, not as a separate or independent program. So basically the same as EasyCodeIt because it officially supports both Windows and Linux with a single codebase... as opposed to having separate codebases/programs for both Linux and Windows.
TheSaint Odd in some ways, that 7-Zip has taken this long to start properly supporting Linux.
Not really though, maybe Igor didn't really bother with Linux until shit started hitting the fan with Windows 10... and maybe now he has finally seen the light and realized that Linux is the only way 😛
Seriously though, there wasn't much demand for 7-zip support in Linux, most people are happy with gzip in Linux land... and also with the newer zst compression rising in popularity recently.
I guess the reason 7-zip was never widely adopted in Linux because it does both archiving and compressing, something which doesn't really fit well with the Unix philosophy of having a single tool for a single job.
tar is the format for archiving files and directories (packing them into a single file) and
zst for compressing the file itself... very modular in approach. You can also skip the archiving bit if you just want to compress a single file, or vice-a-versa if you don't want compression.