That's a container, not a plugin.
And I rather use Midnight Commander that is already included in the base Unraid (mc at the command line).
As for the list, there is a good third of those that I don't use. I would rather not have the core OS with more and more feature that I don't need taking up RAM.
Even if my server has plenty of RAM, many users have the OS run on older platforms or simple HW with 2 or 4GB of RAM. Adding everything would increase the RAM footprint and make it impossible for those systems to run the OS.
Also, most of those are created and maintained by people that are not Unraid employees. That would make the integration more complicated.
Finally, and as mentioned by SimonF, including them in the core OS would prevent fast updates of the plugins for new features, bugfixes, etc.