If there was an appetite for Super Rugby on FTA then the competition could easily ensure there was a game at the same time slot for broadcast every single week of the competition that featured an Australian team.
It just isn't popular enough to fill a primetime slot on a main FTA station.
On the flipside, it's worth too much to be sold for nothing to be shown on a secondary channel.
I think the root of the issue is that the price required to offset the loss in the Foxtel broadcast rights makes it an unattractive option for any broadcaster.
Quantity
NRL and AFL have 8 games a week to manipulate for prime time TV, Super Rugby typically only has 2 or 3 games been played in Australia on any given weekend. Foxtel is really paying for the 2 or 3 games that it can play in the 7:30pm AEST time slot. The kiwi games boost the value as it provides back to back content, but once again the prime time slots is what Foxtel is really forking out the money for..
Value of time slots
If you were able to weight the value of each of the time slots in the the broadcast deal(which I'm sure Foxtel have done) you could argue that 50% of the deal is for the prime time games, 30% is for the 5:30pm games, 10% is for the finals, 5% for the occasional 9:30pm game and 5% is for the South African time-slot games. Thus taking 1 prime time game from Foxtel and moving it to FTA has a proportionately large impact on the overall broadcast rights.
Control of content
On top of all that, it comes back to an issue of been unable to control the content, broadcasters want to be able to dictate the teams who are playing. Its no secret that the Reds and Waratah's rate significantly higher then the Rebels and Force and this complicates the issue for deconfliction when selling the product, the ARU can't guarantee 'X' number of Reds or Waratahs game and this undermines the financials of the deal.
Weeknight broadcasts
One key reason for recent jumps in the value of AFL/NRL broadcast rights has been the move to broadcast on Monday and Thursday nights which offers up to 4 nights of prime time product a week, Super Rugby is unable to do this due to the different markets it caters to but also the issue of travelling and turnaround times.
All of those points listed above harp back to my initial comment that Super Rugby isn't designed for FTA, it suits Foxtel as it provides the prime time games as well as plenty of live content.
NRC is a comp which could be manipulated for FTA Tv requirements, Super Rugby isn't.