In Arma 3, it is possible to set different push-to-talk keys, in the controls. Also, it is possible to double-push one key to have the chat on that channel perpetually activated.
The fact that these options are missing, in a game so fast paced as Project Argo, makes it too difficult for people to switch between direct communication and the different channels and the result is that nobody uses direct communication, really.
A possible fix is to add the option, in the controls, to set different push-to-talk keys for different channels or to have direct comunication activated by default and add the option to disable this, in the game settings.