Trials aren't added early on because a lot of the initial purchases are from people who won't play the game long term. I played for about 2 weeks and quit. If there had been a trial I wouldn't have bought it.
However, as the game gets older there are less people buying it on impulse and the trial turn over starts to benefit the company's sales.
I won't go back to Final Fantasy 14 but I do have an unexplained desire to play Fire Fall.