He may have wanted his final Horcrux to be from a significant death.
Since this was to be his sixth and final Horcrux, making the total number of his pieces of soul seven, which he considered to be the most powerful number, he may have wanted to use the death of the boy who was said to be his downfall. In addition, he already had five Horcruxes by that time, so he had enough to ensure his immortality, so would have had the luxury of a bit more time to save his final one for a significant death.
He also might've needed to find a suitable object to be his last Horcrux.
The objects he chose to make into Horcruxes were arguably more important to the Dark Lord than who he killed to make the Horcruxes. This would make sense, as he was choosing objects to keep a piece of his soul in.
““But Lord Voldemort use tin cans or old potion bottles to guard his own precious soul? You are forgetting what I have shown you. Lord Voldemort liked to collect trophies, and he preferred objects with a powerful magical history. His pride, his belief in his own superiority, his determination to carve for himself a startling place in magical history; these things suggest to me that Voldemort would have chosen his Horcruxes with some care, favouring objects worthy of the honour.’
‘The diary wasn’t that special.’
‘The diary, as you have said yourself, was proof that he was the heir of Slytherin; I am sure that Voldemort considered it of stupendous importance.” - Harry Potter and the Half-Blood Prince, Chapter 23 (Horcruxes)
The idea that his Horcruxes were all significant objects, as well as Dumbledore's theories on what they were, is conclusively proven when Salazar Slytherin's locket, Helga Hufflepuff's Cup and Nagini were proved to be Horcruxes.
“So, the other Horcruxes?’ said Harry. ‘Do you think you know what they are, sir?’
‘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 artefacts have disappeared around him.’
‘The locket!’ said Harry loudly. ‘Hufflepuff’s cup!” - Harry Potter and the Half-Blood Prince, Chapter 23 (Horcruxes)
From what we know about the Dark Lord's creation of his other Horcruxes, he had likely searched for just the right objects to encase a piece of his soul. It's likely, especially in the case of his later Horcruxes after he had already created the ring and the diary, that he created these Horcruxes as he found suitable objects for them. Dumbledore thinks his goal had been to track down one object for each of the four founders, and from what the Horcruxes actually were, we know he had successfully acquired objects from at least three of them.
“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.” - Harry Potter and the Half-Blood Prince, Chapter 23 (Horcruxes)
While we don't know what the final Horcrux would have been, though we know it wasn't Nagini as he didn't have her yet, and we know it wasn't the Sword of Gryffindor as its location was known, he likely would have chosen it carefully. In addition, as he already had five Horcruxes then, he could feel secure in the knowledge that they would protect him from death as he sought the object that would be his final Horcrux.
However, it's not true that his Horcruxes were all made from important deaths.
In an interview, JKR gives a complete list of the people that the Dark Lord killed to create each of his Horcruxes.
Lady Bella: Whose murders did Voldemort use to create each of the Horcruxes?
J.K. Rowling: The diary – Moaning Myrtle. The cup – Hepzibah Smith, the previous owner. The locket – a Muggle tramp. Nagini – Bertha Jorkins (Voldemort could use a wand once he regained a rudimentary body, as long as the victim was subdued). The diadem – an Albanian peasant. The ring – Tom Riddle Sr. - Web chat with The Leaky Cauldron
Of those, only the death of Tom Riddle Sr. seems like it would be particularly significant to him. It's highly unlikely that killing Myrtle, an Albanian peasant, or a Muggle tramp were particularly significant to the Dark Lord. Perhaps Myrtle might have been significant as his first killing, and as a direct result of his ability to control the basilisk, but Myrtle herself was fairly unimportant to his life and plans. Even Hepzibah Smith, a decendant of Helga Hufflepuff, and Bertha Jorkins, an employee at the Ministry of Magic, were unlikely to mean much in his grander plans for immortality and power.
Then why did Dumbledore say the Horcrux kills were all significant?
Remember, Dumbledore saying that all the Horcrux murders had been significant was just his theory, and in addition, whether it was true or not was largely irrelevant to Dumbledore's goal. Knowing the Horcruxes would be significant objects enabled Dumbledore to be able to figure out what they were in order to destroy them. Figuring out who the Dark Lord killed to make them wouldn't have helped towards finding them, so it's likely that Dumbledore prioritized other things over figuring out who he killed to make each of the Horcruxes. Dumbledore did know that the Dark Lord had killed Tom Riddle Sr. to make one of the Horcruxes and suspected that he wanted to make his last one from killing Harry, and in addition knew that he chose significant objects to be his Horcruxes, so Dumbledore may have extrapolated that the deaths would have been significant as well. If this was untrue though it really made very little difference. Dumbledore had been right about all the things that were important to actually finding the Horcruxes.
