Possibly because Libertarians take a consistent small gov't position and the GOP claims to be the party of small gov't.
Don't flame me if you don't understand the humor in what I've written. Just accept it as humor and move on.
If the Republicans had cut the rate of federal government growth, maybe the Libertarians and/or a few more conservatives would have voted for Thune. The fact is that the rate of federal spending has increased in the 2001-2002 period. If the Republicans would defund the Left, at least 100 billion could be cut from the budget.