system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: what are the different versions of Linux and why do they exist?

there are so many versions, such as Ubuntu, Mint, Unity, Gnome etc.

why do they all exist? why are there so many and not just one?
gpt
Unity and Gnome aren't version of Linux, they're desktop environments for Linux. Ubuntu uses Unity, and numerous distributions of Linux use Gnome by default.

Different ones exist to fit different preferences of the creators and to fit different use-cases. 

Ubuntu is a corporate-backed desktop Linux meant for workstations and adapted also for home-use. Mint is very Ubuntu like but the Mint devs fix some of what they see as faults in Ubuntu. 

Redhat is a corporate-backed server Linux that, while it can be used on workstations, is focused on the server market. Fedora is a community project led by the Redhat folks to test new features and softwares they may put into the much more change-averse Redhat distributions.

CentOS is a rebuild of Redhat with all the copyrighted materials of Redhat removed so it can be redistributed for free.

Damn Small Linux aims to never exceed a 50MB core. CoreOS is meant to act as a sort of hypervisor to many docker instances. DD-WRT is meant to be a firmware/OS for router hardware. Many more aim to do many other things.