I think it's just that they are not a popular fish and therefore are'nt collected more so than than them being rare.I have been looking for one for so long that I've forgotten that I wanted one.
The fish is fairly rare to find for sale. And with the fish being rare, makes the price rise. But the fish isn't very... "pretty" or colorful... so I don't get why else it would be so expensive.
I've only known of one place that sells them.