Scale. Cost per employee. Diversity of rackets.

And.... I'm all out of ideas already.

"And, indeed, what is the State anyway but organized banditry? What is taxation but theft on a gigantic, unchecked, scale? What is war but mass murder on a scale impossible by private police forces? What is conscription but mass enslavement? Can anyone envision a private police force getting away with a tiny fraction of what States get away with, and do habitually, year after year, century after century?"

Murray N. Rothbard, For a New Liberty: The Libertarian Manifesto