Simple, supply and demand.
Large fish are difficult to breed, need large ponds occupying large areas of land. To harvest the eggs you need to catch the parents out of the pond at the right time to strip the fry from the parents mouth. Then the tender young fry with egg yolks still attached have to be raised up to a size where they are able to be microchipped and issued with a certificate of authenticity to show that they are second generation captive bred fish (and therefore not putting pressure on wild stocks). They then are passed from the farm to the distributer in singapore to the importer to NZ and finally to the LFS, all who put their mark up on the fish, to cover the costs they incurred. Thats the supply part of it, then there's the demand from superstitious asians who like the red colour and good feng shui it brings, along with something to do with the dragon shape.