Once he found out that the diary was gone, why didn't Voldemort make more Horcruxes?

He wasn't sure that he was capable of making more Horcruxes.

The full quote from Half Blood Prince:

"Does Voldemort know when a Horcrux is destroyed, sir? Can he feel it?" Harry asked, ignoring the portraits. "A very interesting question, Harry. I believe not. I believe that Voldemort is now so immersed in evil, and these crucial parts of himself have been detached for so long, he does not feel as we do. Perhaps, at the point of death, he might be aware of his loss... but he was not aware, for instance, that the diary had been destroyed until he forced the truth out of Lucius Malfoy. When Voldemort discovered that the diary had been mutilated and robbed of all its powers, I am told that his anger was terrible to behold." "But I thought he meant Lucius Malfoy to smuggle it into Hogwarts?" "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.

The quote states that Voldemort didn't think he was able to make more Horcruxes. Voldemort found out about the diary only after he came back to life and met Malfoy. In that time he already split his soul into 7 pieces. Perhaps he thought that splitting his soul again would be unsafe, or that it would destroy the power of 7 fragments of soul, even if one of them is destroyed.