Lucius had planned to give the diary to the Weasley girl, when she started school. It would have allowed him to kill two birds with one stone: bring back Tom Riddle, and discredit Arthur Weasley.
However, now he had a new target in mind.
His son had told him about a Muggle-born Gryffindor, a quite brilliant child by all accounts.
Brilliant, perhaps. But also lonely.
A Muggle-born girl with no friends at Hogwarts, and no family who could receive an owl. Clearly, she had no social skills to speak of.
Who better to be the subject of his little… experiment?
He would send the diary to Draco directly.