I love that people seem to speak as if Valve didn't know exactly what it was doing when it developed this game. They're one of, if not the most knowledgeable companies on the VR market, they have a load of data that should tell them how their game will perform.
They knew this and did it anyway, because presumably - their primary goal isn't just to sell as many software units. It's to elevate the medium of which they've put significant investment into and continue to.
It's weird that people seem to so easily 'get' first party console exclusives, but don't seem to get this. By the same definition, it's a "wasted market opportunity" for Sony to only release The Last of Us 2 on PlayStation, but they do it because there are other benefits beyond just software units sold.