....Because, generally speaking, Libertarians have no particular patriotism.
This is perfectly well-reflected in their economics: Me Me Me Me, screw You You You You.
Libertarianism, rightly understood, is actually a "watchmaker God" theo/philosophy--but its adherents are mostly agnostic or atheist.
That should be enough reasons.
Additional Point: What is so wrong with wanting a weak and ineffectual government? Weak governments generally don't pry into your business and don't tax you excessively.
What ever happened to "you mind your business and I will mind mine?"
The Libertarian/SoCon rift will tear the Republicans apart mark my words...... After all Fascists and Communists both had the same goals but just came from opposite directions.