One of main reason is this game is made by independant studio with a little crew of devs to release it.
Showing the game as it is at this stage will be a "disaster" because majority of people will only see a low res, outdated or a bugged game whithout thinkng the game is still under development and they will bash it quiclky and without mercy (internet is without mercy). This behavior will kill the game before its release and final stage. (recent ie : DoW)
The hidden part of the forum allow us (alpha testers) to communicate freely with devs on what is good and what is wrong whitout the bashing side. If they show this part of the forum, the NDA as no more interest.
... and tactically this increase your interest for the game !