First, I think you misunderstood one thing, it's not a beta game, it's an alpha. So it's way earlier than a beta and yes some part of the game are less polished probably when you go to the end of it.
Secondly, you seems to forget a third option :
As the story is very important in this type of game, it's fairly possible, and I think it's the case here, that Larian doesn't want spoiler on the last part of the game for the person like me who want to wait for the official release.
Plus, in my opinion, it's rude to call a 15h gameplay a simple "demo" when most of the game nowadays is completed in less time than that...
Finally, I'm no developer, but I think in alpha stage, you mostly want to test the core mechanics. The story or other "less" important bug must be corrected later in the development.
