Because they needed five teams to fulfil their part of the broadcast agreement.
Nice try at a viable defence of the ARU's policies. Only problem is: if you hungar for income - as did the ARU in selling broadcasters a fifth Aus Super team - and just provide volume to get it, you must inevitably provide decent and consistent product quality to sustain the viability of the volume you've sold.
That product quality provision from the extra Super team volume didn't occur; the ARU didn't know how to make it occur. So they really should never have sold it, the broadcasters effectively got conned.
The rest is history. 2017: Let's cull volume as the volume we now have loses too much money. Why? Mostly: the quality of much of that volume is crap.
Motto: don't sell more sports product volume if you can't deliver the product quality to sustain it in terms of positive results and fan attractiveness.
So, given the nature of our ARU's talents, or the lack thereof, they should never have started the Rebels, and arguably not the Force either.