Because Europe. They dominate there. In North America, I'd say XBox is still the (slightly) bigger brand. I'm Canadian and I'd say it's like 55%XBox 45%PS4 among people I know. MS botched the launch a bit with One, mostly with the pricing. But since that was corrected most people are simply upgrading their PS360 to whatever they owned before. People like their online accounts.
Saying it's about the games is silly. The vast majority of buyers of either MS or Sony want to play the multiplats. Halo, Uncharted, etc are secondary titles to COD, FIFA, Madden, NHL, GTA, etc.
Nintendo is a whole other story. Their image is terrible and their product doesn't offer the third party mega hits. Combined with the online accounts from gen 7 that gamers want to keep, they have a hard road ahead converting customers. They just aren't in the discussion at the moment.