Titanfall 2 turned a lot of the people that loved and supported the first game off because they threw away so many fundamental gameplay systems that made the game fun and unique in order to try and cater to the COD crowd, and also the new emerging trends like "hero" classes. I absolutely love TF1, probably in my top 3 games of all time, but hated the shit out of the TF2 multiplayer tech test, and was severely disappointed in the final game even after they made a few much needed changes.
TF3, well unless they basically throw away everything from TF2 and go back to TF1 as a base and then add new things to it without removing things, I'm not even going to buy it. TF1 is just a better game in almost every way. TF2 was them chasing players that didn't want to and were never going to play TF2 no matter what they did.
People can praise the single player campaign all they want, but that's not why the vast majority of people buy these type of games, especially not when it's the sequel to a massively hyped and massively successful online multiplayer shooter that didn't even have a single player campaign. They dropped the ball on the MP, that's why the sales were disappointing. The "tech test" they released to the public left a horrible taste in peoples mouths. I didn't even buy it on release it was that bad.