I'm guessing some combination of:
-It's what they want to test right now; and
-It's what's ready to test right now.
We don't have patch notes yet, so we don't know that there isn't more story stuff. They are probably working on story and mechanics in parallel, but if it were me, I would make sure the basic mechanics were ironed out before I spent a ton of time on the details of the story. Druids in particular are a bit of a swiss army knife - they can do a little bit of everything; so having them in the game is useful from a mechanics testing standpoint.