They actually did, as part of DARPA's Warrior Web program [1]. They ultimately spun off the program to Ekso Bionics as a consequence of the Google acquisition back in 2014 [2].

[1] https://www.army.mil/article/125315/darpas_warrior_web_proje... (the first image is of BD's system; you can just make out "BOST" around the center of the left thigh)

[2] https://www.globenewswire.com/fr/news-release/2014/10/02/670...