According to Philly.com, the main reason why WWE chose Santa Clara over Philadelphia for WrestleMania 31 is because The Levi's Stadium will host Superbowl 50 in 2016, and WWE doesn't want to run WrestleMania in Santa Clara after Superbowl 50.

In related news, the following names are confirmed to be appearing at tomorrow's press conference, during which WWE will officially announce Santa Clara and Levi's Stadium as the site of WrestleMania 31:

Santa Clara Mayor Jamie Matthews, San Francisco 49ers CEO Jed York, Vernon Davis of the 49ers, Vince McMahon, Stephanie McMahon, John Cena, Alberto Del Rio and other WWE stars.