If I had to take a stab at why TF doesn't look next gen or have all the bells and whistles like CoD and BF4 is probably due to timing. Looking back, CoD 4 was bare bones too.Yeah, I think if there's one problem with Titanfall, its the lack of making the player feel like they're playing towards something. New player models, weapon camos, weapon attachments, and maybe even armor attachments for your Titan would go a long way in adding a sense of depth to the proceedings.
On the other hand, the game play is fantastic, and I wouldn't change a thing about it. So ultimately my gripes are purely cosmetic, but I do think they would add something tangible to the overall enjoyment for many people, not to mention add to the games longevity.
The head guys made the studio in early 2010 after getting banished from Activision. It probably took them till the end of 2010 to get the studio nicely up and running with staff and having the foundation of the game laid out. That leaves about 3 years and a bit to finish the game on 360, X1 and PC.
They also said they chose Source due to it's familiarity. And it's probably cheap too compared to paying for UE or some other fancy engine. I'm no tech whiz, but when it comes to whiz bang visuals, even I know Source is never brought up.
So as the studios first game, it's currently bare bones compared to the established shooters, but a solid first game. A game that will be played for freshness and gameplay over visuals and customization or bells and whistles.