Seihaku on 09 June 2015
Because the PS4 got out the door first, was cheaper, and didn't have the massive wave of negative publicity that the Xbox One had pre-release thanks to knucklehead executives. It's a slightly more powerful console, but I think that has less to do with it.
By the time the Xbox One was unbundled and cheaper the damage was already done.
That's for the US at least, I think the Playstation has always been a little more popular in Europe and Japan has always ignored the Microsoft consoles.
