Because (see last letter)...



But seriously, I think it was was reasoned in their dialogues? Not sure if I should write more, in case you haven't finished the chapter completely yet.

Silaf wrote: »

In the main storyline of High Isle the 3 faction leaders encounter our avatar and all 3 recognize him as the hero who saved their respective faction.

If our character is actively a member of one of the 3 factions doesn't that make us a traitor? How come the other leaders are fine with it?
And why in the story the faction leaders are risking their life to save each other after fighting to the death in Cyrodiil? I'm confused...

The general story of ESO is... well, it lacks progression. Everyone, or at least the 3 factions, are still ( kinda ) locked in their conflict, despite various things happening and an obvious warming of relationships even before High Isle and the peace summit ( in Summerset for example ). So, weird situations are bound to happen.

But to try to answer your initial question, it may be a reference to the event just before the expedition in Coldharbour, when the mages guild summoned the leaders of the 3 factions in that island to prepare action against Molag Bal, who try to eliminate them. During this event, our character effectively saved all 3 leaders and, in a way, theirs factions, even if you're not counting their respective questlines.

[snip] it's easier to just have dialogues in wich NPCs say the same stuff, regardless of what you did or did not, barring some specifics stuffs. I think that in Elsweyr, Razum-Dar may or may not recognise you depending on what quests you did before, but not in Summerset ( I might be wrong thought ). Anyway, even if there is choices and some reactions from the NPCs depending on what you did or did not ( and I really enjoy these things, these references about things you did, people you met, etc ), it's often a bit 'shallow' - and I can understand : heavily writting for a plurality of stories seems to be quite a lot of work and efforts.

[edited for bashing]

For a character who has done Cadwell's Silver and Gold along with their own alliance questline, there's a basis for each alliance leader to consider the player character their alliance's "champion," with whatever timey-wimey explanation for how we could have been in three places at once leading up to the conference on Stirk (where the three timelines essentially converge). That's always been good enough for me, despite the lack of clarity about how exactly Meridia managed it, but I'm not sure if the dialogue is different for characters who haven't done all three base game questlines so YMMV.

The obvious practical explanation is, like Heren said, that there's a limit to how much the writing can be tailored to each character's experiences, and ZOS tends to lean toward streamlining as much as possible even if it creates narrative inconsistencies. So the best we can do in those situations is come up with our own justifications, like maybe by the time your character gets to High Isle they've made enough of a name for themselves as an adventuring hero that the alliance leaders you haven't actively helped still consider you someone reliable that they could probably trust.

Silaf wrote: »

And why in the story the faction leaders are risking their life to save each other after fighting to the death in Cyrodiil? I'm confused...

I think that's actually one of the main narrative threads of High Isle, the three of them learning to put past animosities aside to work for a better future for Tamriel, and that includes moving past seeing each other as mortal enemies.