For me the main reason for PS4's success is that it has has a steady stream of games coming all year, this year it already has Dying Light, Evolve and Order among others. I might not have bought any of them but it still gives the impression that it has lots of games to play. While the Wii U has these HUGE droughts with absolutely nothing of note releasing.
Moreover the Wii U is lacking in two critical genres , RPGs for hardcore gamers and sports games for average consumers.
Its not always about exclusives.