Titan pilots are "predominantly playing Attrition and Hardpoints game modes," Respawn's post explains. "CTF and Pilot Hunter have not been widely used modes on the PC version. Players trying to access these modes had overlong wait times to find other users and it was making for a poor experience."
The post added that, going forward, Respawn will continue monitoring unpopular playlists and adding new ones. Retracted modes won't be on the sidelines forever, either - Respawn says it will "still rotate Pilot Hunter and CTF in for future playlists" and add tweaks to other game types, which the studio is "currently working on."
Even if you find yourself as the sole player of your favorite mode down the road, Respawn has no plans to pull support for playing modes in private matches. As long as you can fill a round's roster with friends or fellow enthusiasts, you should be all set for your preferred brand of Titan-fueled destruction.