Because if eveyone knew Voldie would hide them in better places. At the minute, the only reason so many have been destroyed has been luck and because of Voldemort's arrogance.

If everyone knows about them, whats to say he won't stick an unbreakable charm on one and chuck it in the sea? Stealth is gonna be the thing harry needs to find them all, and then a lot of luck. Defying Fate: Journey of the Offworlder is a good one for that sort of thing.

 

Someone raised the question that 'If Dumbledore knew about the horcrux(es), why didn't he make an announcement after Voldemort's first defeat when Harry was a baby?' It's a very good question, and I'm sure that other people will have their opinions. I think the answer has multiple aspects.

First, I got the impression from HBP (and I don't feel up to going through it again to double check) that Dumbledore wasn't certain about the horcruxes until sometime after Harry started school. I was left with the impression that the diary got him thinking in that direction and he wanted to verify his suspicions (or see them proven wrong) before taking any action, and he would have been doing that discretely in between everything else in the past four years. This would have been difficult, because we were told that a horcrux is an obscure and very dark magical ritual/artifact, and as such, you can't just look them up easily in the Hogwarts library - not even easily in the restricted section.

Second, the Wizarding world has shown a remarkable ability to ignore information that could disrupt and discomfort their own lives. Denial is not a river for them, it's a way of life. They'd rather be frightened and in the dark than know what's going on and stand up against the bad guys. Bad things are clearly Someone Else's Problem, and that shopkeeper - who must have taken at least five years (is DADA required for all 7 years?) of Defense at Hogwarts (or are we accepting the theory that if Hogwarts is called the most prestigious wizarding school in Britain, there must be other magical educational opportunities in the country?) - has no interest in using those skills to defend his store, home, family and person. I mean, look at the Quidditch World Cup - there were how many people visiting? Send a few hundred Reducto's, Stunner's, Petrificus Totalis's and maybe some cutting curses and that handful of DE's would be a memory. Instead, the people run away screaming while the DE's torch their stuff. Every adult has a wand and should know how to use it, they just get too frightened to remember that.

Third, Harry's treatment in the TriWizard and the year after show that if someone tells the Wizarding world an ugly, unwanted truth, they (as a collective) are perfectly willing to try to kill the messenger.

Fourth - Dumbledore has an established history of concealing information that would be beneficial to share. Why does Harry need to live with the Dursley's? (and have we ever had a real reason why Dumbledore decided to send him there? I mean, was it his decision as Supreme Mugwump, something to do with the Order of the Phoenix, or was he just being a meddling old man?) Why hide the thing Voldemort wanted most in the world (the Philosopher's Stone) in a school full of curious children and then have the first layer of defense something as lethal as one of Hagrid's beloved intrustin' critters? Why not bother to tell someone that Sirius Black is - or rather, 'might be' innocent, and things (like a lack of trial?) should be investigated? Why should Harry study Occulomancy, particularly with Snape? (and I refuse to believe that Dumbledore and Snape are the only two wizards that would know such a thing. Why not find someone with a less emotionally charged history to let into Harry's mind?) Why not give Harry some idea of why Voldemort was after him, even if he didn't want to give the precise wording of the prophecy? Why not at least tell Harry 'My dear boy, Voldemort was not entirely destroyed and may wish to kill you as a sign that he is powerful, and therefore you should study hard. I've arranged some additional lessons in defense and dueling for you.' Why, for the love of anything, not tell someone what reason he had for trusting Snape (there must have been something, even if HBP makes a strong case for this to have been a Big Mistake)?

or maybe Ms Rowling just hadn't thought of such things before and is adding them in as she goes, and THAT's why there was no earlier sign, no real preparations.

As for the inital question of why must Harry be the one to kill Voldie? Why, because everyone else is too frightened to fight him, except for a very, very old man who loves his muggle sweets, is getting too old for a prolonged duel, and shows an overwhelming reluctance to kill and love fo giving second (and third, and fourth) chances to people (coughSnapecough, the annoying Malfoy-spawn). Harry must be the one to kill Voldie because he's the one who won't run away screaming or grovel at Voldie's hypocritical feet. (the lying, demented half-blood that he is. Pure-blooded supremacy from a half-blood ruling his minions thorugh fear and power? bah.)

 

That doesn't make sense. Why would Harry want everybody to know that Voldemort still exists? It won't help the horcrux hunt at all; in fact, it'd hurt it because now a bunch of death eaters would be trying to stop Harry.

On the prophecy, it'd be really cool for Rowling to give a hidden meaning like you have. Your interpretation actually makes a lot of sense, and it takes an enormous load off Harry's shoulders because now all he has to do is destroy the last horcrux - great plot bunny. But I have a feeling Rowling won't do that.

On Dumbledore, he's often said that he believes Voldemort is still alive through some unknown ritual. Well, I think that horcruxes would be one of the most obvious rituals to do for Voldemort. Dumbledore should've immediately searched for rituals to ensure immortality. And with his brilliant mind, he should've thought of horcruxes and done a spell to see if Voldemort created one. And if such a spell didn't exist, he should've created one - he's the most brilliant wizard of his age. So he had plenty of time - 3/2 of a decade - to destroy them and make Voldemort's spirit begone.

Realistically, I think Rowling should do more planning for her books; she just seems to plan one book ahead and leave enormous plot holes in previous books when she invents something new several books ahead that she didn't think about before.