Yeah I don't get why that doesn't happen in Wellington or Dunedin either. Similar vibes to Canberra.
At least Canberra makes sense, small focused academy that is right next to a huge rugby catchment they can pilfer off.
I think the Highlanders lost out early to the Crusaders and they've just never been able to consistently compete structurally (and financially) since. There's a lot of evidence that success begets success in competitions like this, particularly when there are limited levelling mechanisms like draft and salary cap.
The number of provincial unions that came together to make up the super rugby sides is a big one here - Crusaders had their initial leg up from being only a few unions with one dominant, while the others tended to have quite a few more constituent parts, with a lot more mouths to feed in them. That was then really cemented when the modern NPC was established 2006, and then Canterbury went on a decade long reign of terror. That may change in time now that success is being spread around a bit more at that level, but it's still 2 NPC teams for the south islander super sides, and 3-4 for the north, reducing their relative cohesion gains.
This is where Super Au may give us a decent leg up in time - single feeder teams drive cohesion much better than any split model (though 2 does still work pretty well if you need bigger top level squads). If we can get to 8-10 games a year for our sides we should start to really see the benefits roll through. It'll take a generation for that to really shine through though.