Titanfall wasn’t originally planned as an Xbox exclusive. Obviously, developer Respawn Entertainment had talks with Sony about bringing their game to PlayStation systems. While a PS4 version never materialized, Sony had proposed a different platform for Titanfall to land on: Vita.
In The Final Hours of Titanfall, Geoff Keighley reveals that Sony had “offered to help Respawn make a PlayStation Vita version of Titanfall” (via Gamespot). This clearly did not happen.
Of course, future Titanfall games may end up on PlayStation platforms, especially with publisher EA reportedly interested in a sequel.