The answer can be found on steam where another person had the same question.
Mr.Nails said:
So, why not just Early Access? : /
"Great question!
The reasoning behind not doing Early Access on Steam was to sort of "soft release" the beta to a crowd that had an understanding of the game and its development. Since the game is available exclusively on our site, we can do a greater effort of informing people on the state of the beta, and also getting users to view our dev-blog and forum.
We're selling fewer copies this way, but we don't risk bad reactions and reviews on Steam from users who don't read or somehow misinterpret the Early Access disclaimers. Pretty much everyone who's bought the game now have been very positive, and understanding about the state of the game and pace of development. On Steam, though, it's very easy to buy on impulse and we'd rather get a bit less pre-order money than have disappointed players!
That being said, it's very likely that we will put the game out on Steam's Early Access at some point, since not everyone can pay through either PayPal or Amazon! Exactly when that will be remains to be seen.
"
http://steamcommunity.com/workshop/filedetails/discussion/92916246/616199347850545109/