Eu posso pensar nas seguintes razões.
1. Voldemort subestimou completamente Harry Potter
“One of us?” jeered Voldemort, and his whole body was taunt and his red eyes stared, a snake that was about to strike. “You think it will be you, do you, the boy who has survived by accident, and because Dumbledore was pulling the strings?”
Harry Potter and the Deathly Hallows, Chapter 36, The Flaw In The Plan
Com Dumbledore fora de cena, Voldemort acredita que Harry se escondeu.
2. Voldemort estava confiante demais
“Accidents!” screamed Voldemort, but still he did not strike, and the watch- ing crowd was frozen as if Petrified, and of the hundreds in the Hall, nobody seemed to breathe but they two. “Accident and chance and the fact that you crouched and sniveled behind the skirts of greater men and women, and permitted me to kill them for you!”
Harry Potter and the Deathly Hallows, Chapter 36, The Flaw In The Plan
Voldemort acreditava que a Horcrux estava bem protegida
3. Ele não sabia que Harry e Dumbledore o conheciam por dentro
Dumbledore:
Well, as you now know, for many years I have made it my business to discover as much as I can about Voldemort’s past life. I have traveled widely, visiting those places he once knew.
Harry Potter and the Half-Blood Prince, Chapter 23, Horcruxes
Harry:
“I think he would have envied anyone who had a key to a Gringotts vault. I think he’d have seen it as a real symbol of belonging to the Wizarding world. And don’t forget, he trusted Bellatrix and her husband. They were his most devoted servants before he fell, and they went looking for him after he vanished. He said it night he came back, I heard him.”
Harry Potter and the Deathly Hallows, Chapter 24, The Wandmaker
4. Horcruxes estavam bem protegidos
Horcruxes estavam bem protegidos ou com amigos de confiança ou eram muito difíceis de encontrar lugares. Além disso, as Horcruxes eram protegidas por encantamentos.
Anel de Marvolo Gaunt:
I stumbled across the ring hidden in the ruin of the Gaunt’s house. It seems that once Voldemort had succeeded in sealing a piece of his soul inside it, he did not want to wear it anymore. He hid it, protected by many powerful enchantments, in the shack where his ancestors had once lived, never guessing that I might one day take the trouble to visit the ruin, or that I might be keeping an eye open for traces of magical concealment.
Harry Potter and the Half-Blood Prince, Chapter 23, Horcruxes
Medalhão de Salazar Slytherin:
“Not here, precisely,” said Dumbledore. “There is a village of sorts about halfway along the cliffs behind us. I believe the orphans were taken there for a little sea air and a view of the waves. No, I think it was only ever Tom Riddle and his youthful victims who visited this spot. No Muggle could reach this rock unless they were uncommonly good mountaineers, and boats cannot approach the cliffs, the waters around them are too dangerous. I imagine that Riddle climbed down; magic would have served better than ropes. And he brought two small children with him, probably for the pleasure of terrorizing them. I think the journey alone would have done it, don’t you?”
Harry Potter and the Half-Blood Prince, Chapter 26, The Cave