He did not
Few people even knew of Horcruxes. The subject was banned at Hogwarts by Tom Riddle's time, and not many books in the library went into any detail.
"No...well...you'd be hard-pushed to find a book at Hogwarts that'll
give you details on Horcruxes, Tom, that's very Dark stuff, very Dark
indeed," said Slughorn.
—Harry Potter and the Half-Blood Prince
Once Dumbledore became Headmaster, of course, all books on Horcruxes were removed from the library.
Most ordinary witches and wizards know little of the Dark Arts to begin with, and then usually only what they learned in Defense Against the Dark Arts. If Horcruxes were not taught, it would be a rare few extraordinary individuals who would be aware of their existence, let alone their nature.
Further, that the diary was a Horcrux would have been obvious only to a true expert on Horcruxes, of which there may only have been a handful. There are plenty of other books that can control those who read them in some sense or another.
“You’d be surprised,” said Ron, who was looking apprehensively at the
book. “Some of the books the Ministry’s confiscated — Dad’s told me —
there was one that burned your eyes out. And everyone who read Sonnets
of a Sorcerer spoke in limericks for the rest of their lives. And some
old witch in Bath had a book that you could never stop reading! You
just had to wander around with your nose in it, trying to do
everything one-handed. And —”
—Harry Potter and the Chamber of Secrets
So even if someone knew about the diary, they would probably not know it was a Horcrux.
The Ministry (and Dumbledore) would have found out about the diary had Voldemort been successful in using it, but they would most likely have believed what Lucius Malfoy did:
"I understand that Voldemort had told him the diary would cause the
Chamber of Secrets to reopen because it was cleverly enchanted."
—Harry Potter and the Half-Blood Prince
Indeed, we must assume that they story Riddle told Harry (that the diary was imbued with Voldemort's memories) was plausible enough to people at the Ministry, etc., since Harry spread it far and wide after his second year, and the Ministry apparently never figured out Voldemort's secret.
There is an additional, rather important fact: Voldemort did not want to use the diary. When Lily's spell deflected his own curse back on him, Voldemort was at the height of his power. Even then, he had not ordered Lucius to plant the diary. He was waiting. Why? Perhaps he worried that, in spite of the paucity of experts in Horcruxes, someone could still make the connection, and so he wanted those most likely to guess his secret (Dumbledore and Slughorn) to be dead before he used the diary. Perhaps he simply wanted to wait until he had gained total control over the Ministry, so that if someone deduced that he was using Horcruxes for immortality it would not be so important. In any case, this is evidence that he perceived some risks inherent in using the diary.
It is even possible that, given that Voldemort had made the Horcrux when he was young, that he had originally planned to use it, but thought better of it (though the risk of discovery was minimal).
As an additional point, he did not tell any of his followers that he had made Horcruxes. He bragged about his general immortality and invincibility, but without giving any details.
“I, who have gone further than anybody along the path that leads to
immortality. You know my goal — to conquer death. And now, I was
tested, and it appeared that one or more of my experiments had worked
. . . for I had not been killed, though the curse should have done it.”
—Harry Potter and the Goblet of Fire
