Because it's a lot simpler to work ingame, don't need to configure multiple different audio input/output settings, don't need to walk new people through configuring software, logging into a server, joining the proper channel, setting up keybinds, and so forth.
It would be set channel 1, push to talk. Set channel 2. Push other button to talk.
Also while we're at it, the ability to mute range would be really, really swell.
Wiley.