There are significantly fewer concerns about symmetric encryption, and while it doesn't scale to the size or budget of a service like Signal, it's exactly the type of thing the military is good at:

Distribute a bunch of physical artifacts (smartcards) across the globe; guard a central facility (a symmetric key exchange center) extremely well etc.

The military can also afford to run its (encrypted or plaintext) communications over infrastructure it fully controls. The same isn't true for a service provided out of public clouds, on the public Internet.