1. That's Valve's prerogative, not the developer's. If they're not partnered with Valve they may not have had the option to enact pre-purchases. [1]
2. The game has no DRM and Steam preorders (in my experience) download the game files so people can play instantly on launch day. (They call it 'preloading'). For a game as highly anticipated as this, it'd likely just be cracked, leaked and pirated the moment preorders came live.