johnynla choices have to be made. I'm sure askvortsov did his research before choosing a library. Another extension could be created for a different editor.
I personally didn't know CKEditor could be used to edit Markdown. I see it's listed on their homepage, but I don't know since how long this has been possible. The library needs good extensibility to add WYSIWYG support to features like mentions.
Most WYSIWYG editors are based on HTML which would require changing how Flarum validates and stores post, and would break other extensions relying on our formatter API.