Why is Ender the Speaker for the Dead? That's simple—because he's dead himself.
Now, we know what you're thinking—Ender isn't dead. Look, there he is, bright-eyed and bushy-tailed, bouncing from planet to planet, spreading justice and dispensing wisdom, being awesomely interactive and interactively awesome.
In fact, you could argue that Ender is the most lively live thing in the book; the most vital, the strongest, the most thoroughly alert and sensitive. Like live critters, he is super-tough (as Grego finds when he tries to stick a knife in him). He responds to life too, in all its forms, from Novinha to Jane to the piggies to the Hive Queen.
In fact, you could argue that it's just because he's so alive, so empathic, so connected to others that he's able to speak for the dead. The Hive Queen or Marcao keels over, and then Ender comes back and hooks them up to his super-alive empathy battery and pumps them full of life-juice. That's why Miro sees Ender almost radiating life and power during the Speaking:
Somehow this ancient man is able to see the truth and it doesn't blind his eyes or drive him mad. I must listen to this voice and let its power come to me so I, too, can stare at the light and not die. (15.164)
That's Ender, looking at life and light and bouncing with vitamins across the eons. Not dead.
And yes, Shmoop has to confess—Ender is not literally, in-the-ground, skeletons-and-dust dead.
But still, Shmoop insists, he is symbolically, metaphorically, deadish.
Consider this: Because of flying around willy-nilly on all those near-light-speed spacecrafts, Ender is more than three thousand years old. If you're more than three thousand years old, you're dust. He's history, folks, a relic of the far, long gone past. The book he starred in, Ender's Game, is centuries past, and folks barely even remember that world anymore. The Hive Queen as a threat rather than a tragedy; religion as something you have to hide, as Ender's father hid his Catholicism—those are moldy old stories.
If anything, Ender is remembered way too well; if he's three thousand years old, people should barely even know who he is, much less remember all the details of his life. Consider the fact that three thousand years ago for us was before Europe colonized the Western Hemisphere; it was before Islam, before Christianity, before even the establishment of the Roman Empire. To imagine what it's like meeting Ender for the Lusitanians, you have to think about how you'd feel if you met David, King of Israel, walking around your neighborhood.
And as if Ender isn't dead enough after three thousand years, at the very beginning of the book what does he do? He takes off from Trondheim, leaving his sister behind with her life and leaping twenty years into the future. It's like he's died and gone on to another world—a third life, if you will (check out the "Symbols" section for more on this third life business).
When the Aradora at the monastery of the Filhes says that Ender is "widowed" by the loss of his sister (10.97), she is suggesting that Valentine is as good as dead. But of course, from Valentine's perspective, it is she who is widowed and Ender who is pushing up daisies. He walks upon Lusitania, but his former community is kaput, and Speaker is very clear that being cut off from community is death. As Pipo puts it, "a person who really believes she doesn't belong to any community at all invariably kills herself, either by killing her body or by giving up her identity and going mad" (1.137).
But Lusitania is a place where dead doesn't stay dead. The planet is a big old ball of rebirth, an embodiment of Ender's insight that "As long as you keep getting born, it's all right to die sometimes" (18.141). Piggies get chopped into pieces and then get up and tree; Novinha's parents die and become venerable on their way to sainthood; the Hive Queen comes out of hibernation and back to life with all her eggs; Miro goes off to space to skip across time; and Ender finds a new community, a new love, and a new life.
He's no longer Speaker for the Dead, but Speaker for the Living, writing the story of Human, who is still alive but changed, just like Ender, who is also still alive but changed. "How suddenly we find the flesh of God within us, after all," the Bishop says, "when we thought that we were only made of dust" (17.500). Ender, all dusty old dust, is transubstantiated into a member of the community. He's no longer the exiled Xenocide, hated for killing the buggers, nor is he the Speaker for the Dead with an alien voice from behind the grave. He's Novinha's husband now and the father of her children; like Human, he's taken root to live.