I think the "why" is as simple as being motivated by the "how".
Zenos is the only character that has the means, motive, and opportunity to force the climax of the Hydaelyn/Zodiark saga. Zenos has the ability to forcibly mantle preexisting Primals, and he has the desire to create, or rather recreate, the greatest conflict in the history of the star. And the story has moved into a position where this will soon be a reality.
The big unanswered question we have going right now is, how exactly is the Final Days seemingly repeating in 6.0? Because for everything else going on, we have no real explanation for that.

The way I see it, Zenos is responsible for doing something to Hydaelyn that's incapacitated her from her position as the will of the star. Possibly he's either already mantled her, or perhaps that giant crystal structure in Garlemald is serving as her prison in some way. With no more will of the star, the Final Days is restarting just where it left off from when Zodiark stopped it. Obviously, nobody else other than Zenos really has either the motivation or the power to do something like this. If not Zenos, then who? Fandaniel? Some other new character introduced out of nowhere?
Of course, maybe that's looking at it the wrong way. You could probably as easily say that the specifics of this climax were contrived for the sake of Zenos rather than Zenos staying around in order to execute it.

I don't know. Maybe they're going somewhere completely unknown and unexpected with him. At the event they very much stressed that Zenos does really, truly believe that WoL is is friend. Maybe towards the end he'll have an epiphany that he doesn't actually want to kill his friend.