Numbers would be the reason. One side of the other, to a degree both, would have longer queue times. Not as much if a deal during prime time but outside of that BGs would be dead.
That's the reason there will never be a choice like that.
If you made it so that the premade queue can start with two teams I’d love it. I don’t want to queue in a premade unless there’s another premade to fight .
But the fact remains, the best BGs are the ones that are premade-only.