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Tom Riddle's goal is to to make a horcrux from each house owner's belongings. He is successful in making a horcrux from three of the owners' items. Why isn't he successful in making the Sword of Gryffindor a horcrux?
“I can only guess,” said Dumbledore. “For the reasons I have already given, I believe that Lord Voldemort would prefer objects that, in themselves, have a certain grandeur. I have therefore trawled back through Voldemort’s past to see if I can find evidence that such artifacts have disappeared around him.”
“The locket!” said Harry loudly. “Hufflepuff’s cup!”
“Yes,” said Dumbledore, smiling, “I would be prepared to bet — perhaps not my other hand — but a couple of fingers, that they became Horcruxes three and four. The remaining two, assuming again that he created a total of six, are more of a problem, but I will hazard a guess that, having secured objects from Hufflepuff and Slytherin, he set out to track down objects owned by Gryffindor or Ravenclaw. Four objects from the four founders would, I am sure, have exerted a powerful pull over Voldemort’s imagination. I cannot answer for whether he ever managed to find anything of Ravenclaw’s. I am confident, however, that the only known relic of Gryffindor remains safe.”
Canon information from Pottermore, or the HP wikia, is acceptable.