dd -- a good point as well. And it may get to the theory that "Liberalism" was a great idea ... for its time. Time moved on, and left Liberalism it in its wake, so to speak.
There's also the Roe Effect, which I believe has an influence, but that's just ol' cranky neocon me.
Joel -- I'm not sure what you mean by that. There have been failures in any political theory that's been put to practice. There were some successes, though not all of them total successes or without unintended (presumably) consequences.
JP