I don't know why on earth the makers of Word Press wouldn't just name a Taxonomies "Metadata" because that's all it is. It's a set of enumerations that you are going to associate with a blog post (a blurb) to help associate or group them. Changing to this understanding to some archaic "Taxonomies" was absolutely moronic.
I think the best answer is that "metadata" is heavily used in other contexts in WordPress. Most people think of SEO metadata first, or developers may think of the metadata stored for posts and users.
