Long wait times are to blame for the disappearance of two popular game modes. The PC version of Titanfall saw the loss of a couple of modes.
Respawn addressed on their blog the reasoning behind the removal of both modes.
Players trying to access these modes had overlong wait times to find other users and it was making for a poor experience. We want to make sure that not only is there compelling content in the playlists we offer, but that the experience of joining a match works well. We’re tracking what playlists players are choosing, and we’re going to continue to look at unpopular playlists and add new ones on both platforms.
Respawn is also working on new modes and updates that they will announce soon. Pilot Hunter and CTF will come back for future playlists. You can still play these modes in Private Matches