In answer to JT's question, "Why aren't more Mormons libertarian?" I think the answer is entirely centered on the Word of Wisdom and the church's stance during the Prohibition Era.
There are two conflicting ideas: one is the idea that an individual should be free of government coercion, and the other is the idea of prohibiting narcotic use. It seems most Mormons adopt a classically liberal (meaning old school liberal, like Burke and Locke) view of government's proper role. If God has given me free agency, the notion goes, government shouldn't take that agency away. This seems tailor-made for libertarianism.
Most Mormons are uncomfortable with libertarianism's acceptance of drug use. As much as it might seem illogical given the church's stance on agency, the fact is President Heber J. Grant was a huge proponent of Prohibition (Heber J. Grant Priesthood manual, p. 157), and regretted that Utah was the deciding state in its repeal. The Book of Mormon is full of stories regarding a righteous people prohibiting actions they regard as morally repugnant and prosecuting those who participate in them.
As much as I sympathize with the notion that we should teach correct principles and then allow for decisions to be made, the church's history in the Prohibition Era gives me pause before pulling the lever for a Libertarian. That's why I'm stuck in this position: I can't vote for Bob Barr, I can't vote for Barack Obama, and it'll be a cold day in hell before I vote for John McCain.