The rebooted MMO's director, Naoki Yoshida, states that that keeping with the subscription model fulfills an original promise for the game and helps the MMO to have developmental freedom.
Highlighted by Polygon, the interview of Yoshida by VG247 explains that Square Enix is determined to keep promises made to players regarding the game.
"One of the promises we originally stated was that we would release the game with a subscription model," says Yoshida, "So to regain the trust of our players we must of course fulfill this promise. Right now we believe that to be more important than any kind of business decision."
Yoshida goes on to describe free-to-play as the path towards investor-debt with development negatively affected. "Final Fantasy 14: A Realm Reborn is a product that is independent to Square-Enix," Yoshida said. "All of the money's coming from us, so we had more time to put it into what we wanted to do. We're not in a rush to pay anyone (e.g. investors) back."
Source: VG247 via Polygon