If you have two cards that are going to talk to each other one needs to be listening when the other is transmitting, if they're off 95% of the time (like the stuff I design) then 95% of the time someone transmits no one will be listening.

You can try and do something smart around synchonising wakeup times nationwide, but that is genuinely hard (and requires a xtal clock to be awake at all times, also burning power)