If I had to guess, it is because the military has, essentially, merged its constituent parts into one. I believe they call it multi-domain warfare. In the past, the army had little to do with the navy which had little to do with the air force. They realized that they would be far more effective if they combined their operational control into one unit such that if an army soldier in the field needed air support, it didn't require contacting an entirely different department with an entirely different command structure. Nowadays, if the boots on the ground need something blown up, approval is rapid and so is the action. In this new system, they now can depend on each other for immediate backup communication.
Besides, to keep MARS working, they would have to hand out encrypted radios to civilians. Not gonna happen. They don't carry the antiquated tech us hams have.