I was reading a interview about this and they were asked why it was VR only, here is their reply:
"And the answer, according to Valve's Dario Casali, is simply that VR controllers and headsets permit a degree of gameplay detail that "we couldn't possibly do with a mouse and keyboard".
One example given by Casali revolves around the sort of interactions possible with a simple door. Sure, you can open it and close it in a more intuitive, immersive fashion in VR, but, Casali suggests, it's the additional interactions around that door - opening it a fraction to peer through the gap and see what's on the other side, firing your weapon through the crack, or even slipping a grenade through onto the floor - that makes for a truly unique sort of experience."
I'm inclined to belive that its more they just couldn't be bothered though. As many games that use mouse and keyboard or even a controller you can do that stuff with the door.
Plus if games that were designed for VR, such as Paranormal Activity and Star Trek Bridge Commander can put in mouse and keyboard controls after release due to customer feedback, then Valve sure as hell can.