Why? I'm genuinely curious. I've never used their product, but their Wikipedia page says they are a San Francisco based company with end-to-end encrypted messaging and video conference calls[1]. And their home page[2] says "Fully encrypted. Enterprise-ready. Private."

At first glance it seems like a reasonable choice. US based and end-to-end encrypted seems to be what the military wants in this case.

[1] https://en.wikipedia.org/wiki/Wickr

[2] https://wickr.com/