I obviously can't speak on behalf of Kooinda, but there is more to packaging than you might think.
From what I can tell, their previous 'six pack' was the pre-glued ones which you have to hand-assemble. I personally dislike these because they are a pain to assemble and they are a pain to maneuver on the shelf because they hook on one-another, but they do offer a presentable branded package that is accessible to people producing small volumes.
I have not seen the exact plastic ones which you speak of, but I know the type and they are popular in other countries. I am sure that for hand assembly, these would be far easier and faster and therefore more profitable. And as for safety, I would put my money on these statistically holding their integrity better/longer than any cardboard variant.
The cluster pack wraps with the locking underneath that we are all used to require large machinery usually financed via long-term contracts on actual packaging, so are only accessible to producers with significant volumes. These wraps can also be a nightmare, the tabs have to form exactly correctly in the machine for them to work. I always see these taped up in bottle shops because they rely on a crisp fold and a little bit of humidity can soften the card and make them instantly spring apart.
Kooinda are probably just assessing their situation in terms of available options, and have chosen one which is best suited to their needs. The downside with their choice, however logical, is that it bucks the trend of what Australian consumers are used to and it may not be welcomed with open arms. Cocko, I am using you as an example here.