There are several reasons for this. First and foremost is the fact that Strauss's own writings are full of obscure and hidden meanings. He dwelled in esotericism making it very difficult to figure out exactly what he was saying. This leads to the second reason, which is that Straussianism has always been realized in what his followers made of his writings.
Philosophers can be directly quoted no matter what corner of "esotericism" they "dwell in." It's done all the time. In fact, by not quoting Strauss directly one could argue that DiLorenzo is guilty of the very offense he and others accuse Strauss of, i.e., not believing the masses can comprehend rarefied philosophical minutiae.
I think it's more likely, though, that the reason this author and a few others I've read lately haven't been quoting Strauss is because they've never read the guy.
I'm no expert myself, but I've read that Strauss' chief value to conservatism is his penetrating dissection of 20th century liberalism and its statist cousins on the European continent, believing as he did that they ineluctably lead to relativism, which in turn descends into nihilism. Any analysis of the man's work which omits this aspect is some kind of deliberate distortion, and therefore suspect.