To start with, I don't limit my choices to books whose authors identify themselves as libertarians, and still less to books by any specific inner circle of libertarian science fiction writers. If a book supports a point of view that's in harmony with libertarian values, I'm willing to vote for it whether its author is an anarchocapitalist like Vernor Vinge, or a Trotskyite like Ken MacLeod, or a British liberal like Terry Pratchett—to name three authors I did vote for in the past few years. I do want some content that's definitely pro-libertarian (such as Pratchett's critique of gun control, MacLeod's advocacy of futures markets in the colonization of space, or Stephenson's celebration of currency metals as a safeguard against the thievishness of kings) and no positions flatly opposed to human liberty; but given that, I'll vote for a book that's well written over one that puts forth the straight libertarian party line.
And my standards of "well written" are somewhat literary. I want a writer whose prose is at least workmanlike, and ideally one whose style is reason enough to read them apart from content. I want one who doesn't inflict stereotyped characters, overused plots, or idiot lectures on me; I'm willing to reread Atlas Shrugged and take the speeches as Ayn Rand's equivalent of the arias in grand opera or the martial arts duels in wuxia films, but it's not every writer who can make me sit still for long expository passages. And I'd rather not read fiction by writers who aren't paying attention to what's happening in current science fiction, but are still working in the style of the 1950s or the 1930s. I don't identify libertarianism with conservatism, and that includes literary conservatism; I see it not as nostalgia for an idealized past but as hunger for a freer future—and that looking forward to the future is the lifeblood of sf in general.
Beyond that, I don't necessarily find the most interesting libertarian ideas in the works of self-identified libertarian writers. Kingsbury's Getan culture in Courtship Rite, based on optimization through evolutionary competition as applied to genetics, economics, and ethics, and his advocacy of stoicism as an ethic for a free life; MacLeod's True Knowledge, based on the ideas of writers like Stirner, Nietzsche, and Tucker, and his strange blend of revisionist Marxism with anarchocapitalism; Stephenson's tracing the roots of scientific inquiry, religious tolerance, the gold standard, and the abolition of slavery to the late 17th and early 18th centuries, and his ingenious linking of all of them to the rejection of alchemy—all of those set off sparks in my mind; they make me think new thoughts and ask new questions, in the same way that Vernor Vinge (my favorite living libertarian sf author) does.
In my view, the Prometheus Award and the Hall of Fame are reading lists. If I meet a libertarian who's never read sf, or an sf fan who doesn't know about libertarianism, I'd like to be able to give them a list of books to read that will get them interested. If I refer them to the LFS Web page, most of the books they'll find there are suitable. But to keep it that way, I have to vote for books that even someone who doesn't agree with libertarianism could find worth reading (just as I find China Miéville's Perdido Street Station and The Scar worth reading despite disagreeing with Miéville's politics and economics); not for books that only someone who already agrees with libertarianism would want to read.
Of course, that's only my point of view; by itself, it doesn't explain why a lot of books that I favor have won the Prometheus Award lately. It isn't as if I had dictatorial authority over members of the LFS, or as if anyone were likely to. I like to imagine that I've been picking out books whose merits are sufficiently apparent that a bunch of libertarian science fiction fans are able to agree on recognizing them. It may even be true, though I have no indepenent basis for confirming it.