Professor Dumbledore gives his guess to this in Harry Potter and the Half-Blood Prince chapter 23.
[Harry:] ‘But I thought he meant Lucius Malfoy to smuggle it [the diary] into Hogwarts?’
[Professor Dumbledore:] ‘Yes he did, years ago, when he was sure he would be able to create more Horcruxes, but still Lucius was supposed to wait for Voldemort's say-so, and he never received it, for Voldemort vanished shortly after giving him the diary. No doubt he thought that Lucius would not dare do anything with the Horcrux other than guard it carefully, but he was counting too much upon Lucius's fear of a master who had been gone for years and whom Lucius believed dead. Of course, Lucius did not know what the diary really was. I understand that Voldemort had told him the diary would cause the Chamber of Secrets to reopen, because it was cleverly enchanted. Had Lucius known he held a portion of his master's soul in his hands he would undoubtedly have treated it with more reverence – but instead he went ahead and carried out the old plan for his own ends: by planting the diary upon Arthur Weasley's daughter, he hoped to discredit Arthur, have me thrown out of Hogwarts and get rid of a highly incriminating object in one stroke. […]’
As Jason Baker points out in his answer, Professor Dumbledore explains a bit more of Lucius's motivation, especially about discrediting the Weasleys, in Chamber of Secrets chapter 18, in a thinly veiled hypothetical manner because he doesn't have enough evidence to prove Lucius's guilt.
Mr Malfoy went even paler than usual, but his eyes were still slits of fury.
‘So – have you stopped the attacks yet?’ he sneered. ‘Have you caught the culprit?’
‘We have,’ said Dumbledore, with a smile.
‘Well?’ said Mr Malfoy sharply. ‘Who is it?’
‘The same person as last time, Lucius,’ said Dumbledore. ‘But this time, Lord Voldemort was acting through somebody else. By means of this diary.’
He held up the small black book with the large hole through the centre, watching Mr Malfoy closely. Harry, however, was watching Dobby.
The elf was doing something very odd. His great eyes fixed meaningfully on Harry, he kept pointing at the diary, then at Mr Malfoy, and then hitting himself hard on the head with his fist.
‘I see …’ said Mr Malfoy slowly to Dumbledore.
‘A clever plan,’ said Dumbledore in a level voice, still staring Mr Malfoy straight in the eye. ‘Because if Harry here –’ Mr Malfoy shot Harry a swift, sharp look, ‘and his friend Ron hadn't discovered this book, why, – Ginny Weasley might have taken all the blame. No one would ever have been able to prove she hadn't acted of her own free will …’
Mr Malfoy said nothing. His face was suddenly mask-like.
‘And imagine,’ Dumbledore went on, ‘what might have happened then … The Weasleys are one of our most prominent pure-blood families. Imagine the effect on Arthur Weasley and his Muggle Protection Act, if his own daughter was discovered attacking and killing Muggle-borns. Very fortunate the diary was discovered, and Riddle's memories wiped from it. Who knows what the consequences might have been otherwise …’
So the Dark Lord did not instruct Lucius to dispose of the diary, but he carelessly revealed just the wrong information to him that Lucius went after his own head while the Dark Lord was fallen.