Well, mostly because he wanted the Chamber opened. He wanted to get someone into that place, he wanted to give them the diary that would take them into it.
'[...] What intrigued and alarmed me most was that that diary had been intended as a weapon as much as a safeguard.'
'I still don't understand,' said Harry.
'Well, it worked as a Horcrux is supposed to work - in other words, the fragment of soul concealed inside it was kept safe and had undoubtedly played its part in preventing the death of its owner. But there could be no doubt that Riddle really wanted that diary read, wanted the piece of his soul to inhabit or possess somebody else, so that Slytherin's monster would be unleashed again.'
'Well, he didn't want his hard work to be wasted,' said Harry. 'He wanted people to know he was Slytherin's heir, because he couldn't take credit at the time.'
'Quite correct,' said Dumbledore, nodding.
Harry Potter and the Half-Blood Prince - p.468 - Bloomsbury - Chapter 23, Horcruxes
It's not much of a hiding place, if you yourself are going to give some future student the keys.
Second, you need to appreciate that Voldemort is convinced that he alone knows of the Room of Hidden Things. He is the heir of Slytherin, he believes Hogwarts is his birthright and that he alone knows its secrets, that to him alone has Hogwarts revealed its secrets. He is arrogant enough to believe that nobody else could have found the Room of Hidden Things and isolated enough never to have heard others mention it.
As for the school: he alone knew where in Hogwarts he had stowed the Horcrux, because he alone had plumbed the deepest secrets of that place ...
Harry Potter and the Deathly Hallows - p.444 - Bloomsbury - Chapter 27, The Final Hiding Place
Again:
Tom Riddle, who confided in no one and operated alone, might have been arrogant enough to assume that he, and only he, had penetrated the deepest mysteries of Hogwarts Castle. Of course, Dumbledore and Flitwick, those model pupils, had never set foot in that particular place, but he, Harry, had strayed off the beaten track in his time at school - here at last was a secret he and Voldemort knew, that Dumbledore had never discovered -
Harry Potter and the Deathly Hallows - pp.498-9 - Bloomsbury - Chapter 31, The Battle of Hogwarts
Finally, it seems to me that it may never have occurred to him to do so. That Chamber was great-great-great-...-great granddad Salazar's place. It had his statue, it housed his monster and it was about his plans for the school. Voldemort was proud of his ancestry and keen to reopen the Chamber and realise his great ancestor's plans. But it may not have occurred to him as the right place for his Horcruxes.
And hey, with Voldemort in charge and Hogwarts as it should be, the Chamber of Secrets would have no need of secrecy. He might even want to open the place up. Voldemort made his Horcruxes in order to realise plans for the wizarding world and Hogwarts school.
