I think because a number of possible reasons:
-It's a new franchise (in a genre that is saturated)
-It's not available on all platforms
-The game does not cater to new/bad players
-The game doesn't have a lot of content or fleshed out multiplayer features
-There are a number of performance issues for a lot of people
-A lot of people are not prepared to spend a large chunk of their time while playing Titanfall, waiting in the menus.
Most of those are post-sale impressions which might reflect on the number of active players, but not why sales haven't caught fire.
Out of those, the first two are the big factors next to what I believe is the biggest: poor marketing. Everything about this game was Xbox One. Sure, there were press announcements and a line of fine print on banners mentioning a 360 version, and the PC version had the beta and some attention within the hardcore audience.
But to the general populace, that COD audience that is being targeted, it was an Xbox One title requiring purchase of a $499.99-$449.99 system in the US that just went through 6 months of PR hell and losing a lot of mindshare to a cheaper and more focused competitor. That's also on top of requiring the use of Xbox Live Gold, another $59.99 investment (what the majority pay, so buzz off with the $40 sale tag arguments to those who do so) for the unknown.
Maybe Titanfall wouldn't have done much better if a fair shake in marketing was given to the other two platforms supporting it, but given the last few NPDs, months of silence, and shaky player pool I'd say someone dropped the ball. I'm not sure how a second installment will fare when by the time it releases, a certain established franchise could adopt plenty of mechanics to make it a moot point in uprooting to the newcomer.