The vast majority of PC games use DirectX. DirectX has more than driven the market for the past decade, it's defined it. All major GPUs' feature set were defined by DirectX, with OpenGL largely trailing behind. OGL is catching up these days, but most games are still focused exclusively for DirectX.
maybe the market is changing... steam wants to embrace the linux community with its steam-for-linux move. maybe they are betting on the OGL-trail for future games, and the more rapid development through the demand from linux users and hopefully OGL developers to make this the standard and forget about further developing directx for games and grafic cards altogether... maybe steam knows things about the directx development we're only becoming aware of.