Voldemort originally made six Horcruxes because he thought that seven was the most powerfully magical number, meaning that it was best to have his soul in seven pieces. However, during Half-Blood Prince we find out that Voldemort knows that the diary Horcrux has been destroyed, meaning that only at most six pieces of his soul remain.
With this in mind, why didn't he make another Horcrux? If doing so would restore his number of soul fragments to numbering seven, then he would surely find that beneficial for the same reason that he had originally decided to have seven fragments. And if this is not possible, then as he cannot have seven fragments, why wouldn't he make more Horcruxes anyway? As far as he seems to know, if you cannot have a seven piece soul, then there's no disadvantage to splitting your soul even further.