A couple different reasons. Essentially, Microsoft was talking with them about Durango very early on and courting them to move Titanfall over to that. They asked Sony if they could talk to them about the next Playstation, but Sony constantly said they weren't ready to discuss. It even mentions Sony were willing to help Respawn to get a Vita version of the game. It mentions that they essentially pleaded with Sony, saying "Listen, things are about to be locked in behind the scenes. If you want this, please talk to us", which Sony again declined.
It also mentions that at a point later in development, Zampella and West sat down with Riccitiello to tell him that they didn't have enough manpower to develop for any consoles other than Xbone and, maybe, PC, and that Respawn was essentially out of money and the game would need to be delayed and that they'd cut single player. EA ended up having to find the funding for the project with a first party, and Microsoft were willing to put up the money to actually finish the game.
