Why? "Markdown" is not a standard, it's a loose collection of formats. CommonMark is a standard, and they are not calling it that. Anything that is Markdown-like can be called Markdown.
It's the same deal with lisps. Lots of languages are "lisps", even the ones that are slightly different than the ones you're used to. Then there is a Common Lisp that is a language that is also a lisp.
It’s just nice to be able to differentiate between markdowns that adhere to some existing implementation (GitHub, CommonMark or something else) and those that are their own flavor.[1]
“Markdown” is marketing speak for “you probably already know this”. Which yes, sure, but all the little tiny differences between all the different implementations can act as annoying papercuts.
At least it’s not wiki markup...
[1] GitHub Markdown etc. are fine since they can be called that (X Markdown) and not just “Markdown”.
Some snippets from those two pages, where they call their own syntax just "Markdown":
> Markdown is a lightweight and easy-to-use syntax for styling all forms of writing on the GitHub platform.
> You can use Markdown most places around GitHub:
> Here’s an overview of Markdown syntax that you can use anywhere on GitHub.com or in your own text files.
> Create sophisticated formatting for your prose and code on GitHub with simple syntax.
It's until the bottom of those pages that they start mentioning GitHub Flavored Markdown (GFM).
Just like how Obsidian deals with the very same documentation.
Not sure what the problem is, Markdown has always been different on different entities on the internet, since there is no common specification. Blaming Obsidian for this seems short-sighted.
It's the same deal with lisps. Lots of languages are "lisps", even the ones that are slightly different than the ones you're used to. Then there is a Common Lisp that is a language that is also a lisp.