as350nut,
since every 'issue patch' to imperfections of design, manufacturing/parts material quality etc of RHC is financial/time burden of the owner/operator as it looks to me from various issues with blades delam/corrosion/fuel tanks etc, fitting the tanks is costly enough both purchase, downtime and labour to make it 'not sensible' from purely financial point of view without any bigger maintenance on the machine.
I'm not so familiar with Australia/CASA differences to FAA/USA, but the (incidentally) last year crash, also fairly survivable, ended up pretty much same, which 'prompted' the 1yr earlier deadline on R44s all having bladder tanks. I added the link after brief search just today for illustration to those who haven't followed it in the past.
Why wouldn't FAA, EASA or other agency/authority be able to issue AD? SBs are just CYA from RHC's side (pre-emptying the AD to avoid mass demand). Obviously not enough people die and tanks rupture during a crash/rollover. There is track record of survivable crashes with fires killing people, so in view of many, it meets the condition of 'known safety deficiency' as reason to issue AD.
Helicopter manufacturer knew of fatal design flaw, say US lawyers (Another quick search for the last year's crash. Make your own opinion)