The game seems to be set roughly around 3,653 BBY and their combat art was created 3,678 BBY.
Well, the creation of the style was "sometime prior to 3678" so it's not like there has only been 25 years for the style to develop. It's been around before that time; it's just that's the first time that it's been directly referenced in dated canon level material.
It's important to note that the reason we get that year for Teras Kasi is because there is Teras Kasi used in the TOR prequel comic (which was released by BW as promo material for TOR in 2009 apparently, though I can't find it), so it's not like it doesn't exist in this time frame (assuming we can trust the wiki that it actually *is* practiced and not simply a generic martial arts style that is unnamed and assumed to be Teras Kasi). The problem with Teras Kasi isn't the time frame. It's the intent and alignment of the style and how it interacts with the fact that both factions are deeply aligned with their respective Force wielding order. It's hard to write a sensible story for a character that is supposed to be decidedly anti-Force when more than half of the people they're going to encounter as players and a substantial portion of their faction are Jedi and/or Sith.