Respawn's co-founder, Vince Zampella, has said that it is too late to bring the original Titanfall to Steam, but there could be a chance for Titanfall 2 to make it instead.
The original game is currently only available through EA's Origin platform, which has probably put a lot of people off buying the game. Zampella recently spoke to Game Informer and said that at this point in time it would be "a lot of work" to bring the original to Steam. He said that he would love to have done so on day one, and that "we should not make that same decision again."
At some point you look at it and you say is it even worth now this much later the effort to put it on Steam, when it would be a lot of work and kind of bifurcate the community? We would have loved for it to be on Steam from day one, but at some point it just doesn't make sense anymore and you start looking to the future and I think we should not make that same decision again [emphasis added].
The decision to put the game on Origin was clearly down to the fact that EA published the game, but it looks like things are changing for the second installment as it could move over to PlayStation consoles as well as Steam. Hopefully this is the case as the game being on Steam would be a lot better, and some people flat out refuse to buy games from Origin.