Sun Microsystems had hardware and a version of Unix that was POSIX-compliant. This passed the government openness/interoperability test in existence and anyone with authority and budget could buy Sun, and then buy approved applications and run them.

NeXT had a superior development environment, but, Steve Jobs didn't spend the 1 or 2 engineer-years it would have taken to make the OS be POSIX-compliant. Thus, in addition to authority and budget, a waiver needed to be signed off on, in order to buy NeXT hardware.

The "three letter agency" or "TLA" portion of the US government, didn't always need off the shelf apps, but they had to build their own; and they didn't have to pass scrutiny over a waiver for something with a national-security implication.

Thus the CIA, DIA, NSA etc. bought both Sun and NeXT hardware.