The game has been available for a few years now, and due to is using a very old engine version and us being unable to successfully port it to newer versions, updates stagnated.
We feel like it makes sense to make the game free to play, and see if there's a bigger userbase out there interested in the game, and if so, see if there's a new push to make to try and get the game updated.
As soon as Valve approves of the game to become free, we'll do it!