I have a reason, maybe I just haven't communicated it well enough. Time to try again:
So, Paladins. In FF14, the implication seems to be that the Sultansworn — a group that is deeply embedded with the established government — are "good" (with scare quotes), but there simply aren't enough of them to do all the good that's needed to make the world as good as it could be. If only they had more reach! What if wandering adventurers could uphold the values of law and justice in the places where the proper Sultansworn aren't? That's free Paladins: hopeful, optimistic, leaning lawful good, operating within the establishment to take up the cause of the defenseless. The system is overall good, there's just not enough of it, so let's do our part!
Dark Knights are the flip side of paladins, jaded and operating outside of the establishment to protect those the corruption within the establishment preys upon. They have abandoned the system because the system cannot be saved and is not worth saving. The Dark Knight quests provide numerous events where you struggle with this identity and the challenges that go with it: you operate outside of the law and extra-judicially kill those who abuse their power.
Warriors channel a double-edged power they have called the "Inner Beast", and our specific school of Warrior aims to tame that Inner Beast so that it can be used to protect allies as opposed to unleashing wanton destruction. While Paladins and Dark Knights are engaged with political ideas about how best to do a justice in an imperfect system, the struggle of Warriors is more internal: how can I be an asset rather than a burden to those in my life? And when the answer to those questions is found, the result is a powerful rage that provides utility without being a liability.
So we've got these 3 concepts for tanks, each with their own vibes. Let's add a new tank. How is it going to differentiate itself from the tanks that are already there?
Unique weapon? Check. Unique outfit? Check. Origin story for their arts? Check. But what does it mean to be a Gunbreaker? What are they about? And to the point, can it be described in a way that makes it seem genuinely unique relative to these 3 other classes? If there's a good answer to this question, I haven't seen it.