'Are you kidding?' controller asks Southwest jet that landed at wrong airport

Oops... Passenger jet lands at the wrong airport. I'm not a pilot nor an aviation professional. So can anyone with more knowledge on the topic explain how this can happen? I thought these planes had GPS, autopilot, ILS,... Isn't it mandatory to use these systems till the plane is on the ground ? (edition.cnn.com) Plus d'info...