The release date, as everyone is saying, and the fact that the first was an XB1 exclusive. If you make a new franchise exclusive to one platflorm, you run the risk of it not gaining enough mindshare with the general public. Sure, it was a meme among the hardcore "Have you seen Titanfall?" but I think most potential buyers, if they were aware of its existence, didn't have any emotional investment in the series, which is a crying shame.
I don't think games "deserve" sales, but TF2 deserved something better than it got; better treatment from EA at least, that's for sure.