Of course the obvious answer is that its one way of guaranteeing a symbolic win over Conservatives, but there is actually another important strategy in play as well.
By loading the panel with left wingers, and allowing only one (or infrequently two) Conservatives, and with far left Tony Jones as a pseudo “moderator”, the Q and A producers help reinforce the wishful perception that Conservative views are only held by a minority.
It has always been a big part of the left’s game to isolate those who oppose them. They have always wanted the opposition to themselves think they are in a minority.
Its a strategy that has worked well for them for a long time, but now that communication channels are more open, its becoming more widely exposed as a tactic.
Of course the ABC should not be under the control of the left at all, but that is another problem, one that is rooted in the failure of the Liberal Party to do anything to remedy it.