According to a report from our friends at WrestleChat.net, who have been out ahead of the WrestleMania coverage all along, there were two reasons why Santa Clara won out over Philly for hosting WrestleMania 31.1) It was just held in North Jersey and the WWE felt there would be too much overlap. And 2):
Second, the WWE felt like Santa Clara simply did a better job in the pitch process. According to a report from PWInsider, WWE officials got the feeling from Philadelphia and Lincoln Financial Field associates like the city was doing them a favor by allowing them to host their signature event in their town. Santa Clara and 49ers officials were said to have made the WWE feel more invited and showed more enthusiasm in having the show there. In the end, the WWE simply felt like Santa Clara had a much larger interest in hosting WrestleMania 31.