If Austrian economics and mainstream economics were both equally persuasive, we should still expect to see far more libertarians who were persuaded by mainstream economics than libertarians who were persuaded by Austrian economics just for reasons of exposure. But the reality, as Caplan points out, is much different. Again, how did this state of affairs come about? Why is it that such a huge number of libertarians even know about a mostly unknown school of thought, let alone find this heterodox approach to be the best way to analyze economic issues? The explanation that seems most plausible to me is that people just find arguments based on Austrian economics more persuasive than arguments based on mainstream economics.
[Scratching my head...]
Maybe! But I suspect that the real story is that Austrian economics is persuasive to people who were going to become libertarians anyway. It is a powerful meme in the libertarian sub-culture, but it draws strength from - and hurts the spread of - the broader libertarian meme to which it is attached.