#elfangor

Doylistically (out of universe): The mentor who dies passing along his power is a classic SF trope, and Animorphs does a classic job of both playing that trope straight and subverting it. Elfangor becomes the emotional motivation for the team, because now they can't not fight the war — he literally died to give them a chance to save humanity. Given how much the series is baked on the idea of kids having no guidance at all when tasked with saving the world, he cannot stick around to mentor them. He has to die.

It's also not clear that Applegate et al. committed to the idea of morph-healing before #4, because it doesn't come up until then, and the rules of morphing are pretty fuzzy in that first book.

Watsonian (in-universe): Various explanations include:

When the Ellimist re-andalite-ified Elfangor, he never gave his morphing power back (X)

Elfangor has to let Visser Three kill him, or else Visser Three will tear the planet apart looking for him (X)

Elfangor lets Visser Three kill him in order to give the kids a chance to get away (X)

Elfangor is injured with some kind of weapon or radiation poisoning that causes an injury morphing can't heal

Andalites aren't aware that morphing can heal injuries to one's base form (X)

Ever saddle your human girlfriend with the magic fingernails and ride towards the intersection of the memories of three planets? Not yet? Hm. Ah, well, I hope the story has the happy ending they deserve.