It's a badass dude in cool armor is impressive at a glance and does a good job at communicating "fantasy setting!". You'll notice that in Stormblood they used Zenos on more promotional Material than Samurai because Zenos was more visually impressive than a robed man with a Katana at a glance. Then Samurai was more used than Monk because Monk was a red herring and they weren't ready to reveal Samurai as the real poster job yet a samurai is more visually impressive than a martial artist at a glance as well. ARR also had Gaius and Warrior, both armored combatants, Heavensward had Dragoon, and Shadowbringers had Dark Knight.

Other than that though it works on a thematic level. Endwalkers villains are the Telephoroi/Garleans, Zodiark, and the Ascian remnants. The Telephoroi are an evil technological expansionist empire on the attack with a red band black color scheme. Zodiark is an evil god representing darkness, and the Ascians are his evil cult. Paladin is pretty much the antithesis of all of these to varying degrees. Visually it's all heroic colors, Gold, Blue, and Silver/White. Its the stalwart defender instead of the attack. Its magical instead of technological. It's holy instead of dark aligned. Paladin pretty much hits every note that you want it to for the opposite reason that Dark Knight was such a good choice for Shadowbringers.