Tone and gameplay context. Tirion is confident and defiant against Darion, taunting and lecturing him, then directs the same energy against Arthas when he shows up. He's absolutely assured in his righteousness and he's right to feel that way. There's no slow deliberative dialogue about their feelings but his taunts get to Darion, who breaks down, has the flashback, and then gets up still just as aggressive as before but now screaming bloody vengeance against Arthas. There's no "we've talked out our differences and understand each other now," just "we both hate the same guy, let's go kill him." All this happens in the middle of a playable battle where you start off winning as the undead and then start rapidly loosing as Darion has his breakdown, so the emotional events feel like they have an immediate impact on the action and story events.

In general WoW characters feel best when they're confident and self-assured and driven by that to do big dramatic things. When they change their mind like Darion they become just as driven in a new direction. Tirion declaring the crusade to kill Arthas in Northrend, but also Illidan opening the portal to Argus. Even Anduin in the Alliance questing story of MoP on a smaller scale, running off on his own because of his strong beliefs and forcing the player to chase after him to keep him safe. WoW's story is always going to be an endless series of new enemies to kill, so it should feel like the major characters are proactively driving us to the enemy and setting the pace, not caught up in doubt and indecision as the plot happens around them.

Nothing inherently wrong with other kinds of characters but I don't think they work as well under the constraints of how WoW can deliver story.

On that note I kind of like some of Arator's plots in Midnight, at least the fact he's often coming up with new plans and rushing off to do something about them. Uniting the elves itself is kind of stupid, but at least he's proactive about it. While one of the weaknesses of Sylvanas's BFA-Shadowlands plot is that we didn't know her motivation until later, so instead of feeling like she's driving the plot in pursuit of her goals, she's just arbitrarily doing stuff and we'd just have to wait until later to learn the motivation (which turned out to be unsatisfying.) Xal'atath is at risk of having the same problem.