Unstable, unpredictable demand.
Sole-source suppliers.
Throw in long development cycles with possible substantial in-process changes and politicians capriciously changing quantities ordered or cancelling projects and, "Welcome to the joys of being a defense supplier!"
E.G., had many in Congress - probably Dems - gotten their way, the US Navy would have fought in the Gulf War with a mix of F-14s, F-4s, A-6s, and A-7s as their primary planes. No F-18 Hornets.