Well, as someone who owns all three systems, I can't answer the question. But I can say why I do have an Xbox One, and it's for the same reason I owned a 360: Halo. That's the one thing above all others that keeps me coming back. That and all the social connections I've made over Xbox Live after over a decade of playing Halo online.
But had I not ever discovered Halo, I likely would have never invested in Xbox. I would have stuck with a Nintendo-PlayStation combo.