Personally I would say because there are so few of us, shops don't stock the gear, market forces and all that. I purchased my first skimmer overseas, latest one Locally as I wanted local support, Both start with a big D. Local one would have been about the same cost if I imported by the time freight and GST were added.
Our local LFS has been very helpful however they are expensive I have found Bruce @ Deco City to provide excellent service and competitive prices, can supply most equipment to order etc etc. I think there is just not the market here for all the required equipment so many stores aren't interested in stocking any marine stuff, I can totally understand this as they are there to make a profit, its not generally their hobby as well.