"How so?"
Because libertarianism rejects the idea that the power of reason can be employed by the *state* in a top down effort to order and arrange society in the name of fairness, equality, justice, and the common good. Libertarianism, OTOH, maintains that the random actions of free men, acting in their own interests, pursuing their own goals, will produce the greatest benefits for the citizens.
Rationalism was the philosophy that came out of the French revolution and is the root of socialism. Libertarianism is more allied to the English tradition of individual freedom and personal liberty on which our own government was once based. I think we have pretty well moved to the French model these days, however.
Weirdly, there is a parallel here to the crevo threads, with the ID theory resembling the French rationalism and the evo theory paralleling the English tradition. It actually comes up on those threads occasionally. Beside the point, though.
Thanks.