Reddit had a private Q&A with two of the developers, some interesting stuff in it! Added a bunch of new bits and bobs to the notes spoilers in the first post if anyone wants to check it out.
I'd imagine it's just that it's pre-alpha / alpha. There's lots of graphical issues I noticed, like it doesn't appear to use any anti-aliasing, the lightning is off on monsters like some just aren't reacting to light sources, and there's clipping and camera issues. Can't imagine they'd release the game like that, and I'd hope that in 18 months or so (or whenever the game comes out) it'll have came on a long way. But yeah the direction they are taking looks great and I'm glad they've gone back to "serious mode" with the tone of the game, and as per that Reddit Q&A I mentioned above, they are aiming for a slightly slower Diablo 3 pace with a world as grimey and putrid as Diablo 1 and 2.
Here's some HD screenshots though:
https://imgur.com/a/0YxKLSd