This is a very different and complicated question, and requires a more full response, well more than I put here.
The issue is what is available that is compatible in PHP and HTML5. This is not a really clear cut issue. A WYSIWYG editor can carry a much larger footprint than is desirable in Moodle, so the markdown editors are used instead. Agreed, a WYSIWYG editor would be more helpful but performance, I suspect, would take a big hit.
Another issue is that there was a lot of discussion about which editor should be used around the time that v2.0 was released. Two main ideas were pursued, one was adapt TinyMCE or develop Moodle's own. Both happened. A number of very clever people were able to replace TextArea with TinyMCE in v1.8 or v1.9, and, shared it. Worked well, too. From memory, it became a core plugin, so that became the obvious candidate for v2.0 (I might have the timing wrong, but this is how I recall it.) A little later, Atto appeared as the Moodle developed markdown editor to eventually replace TinyMCE. The idea, I understand, was that a core Atto was developed and released and other devs would be encouraged to create Atto plugins for Users to download and install themselves. Unfortunately, this doesn't seem to be happening quickly enough so we are stuck with a seriously underdeveloped Atto or a soon to be dropped TinyMCE.
The issues are a lot more complex than this, I suggest, and like all precis, this is about as accurate as a child's hand drawn portrait, (i.e. its almost recognizable as a drawing ). Hopefully, I have touched on a couple of issues that give some clearer clearer picture for you.