Like Sioux said, it's entirely possible to make money off of nebimonites without a Lu Shang's rod. I skilled up on them from about 19-28 (probably starting a bit early) using crayfish paste on the ferry, and used the money to buy my early-30s RDM spells.
I think the problem with nebimonites is just that they're an absolute pain to fish up as compared to some other fish, and, as far as gil goes, probably won't make as much gil for a lvl 25 fishertypeperson as moat carp or some other fish. As far as I know, the only places you can catch nebs are SSG and the ferry. I've heard the magical level that you won't get aggro in SSG is 59, which makes it an impractical fishing spot for a lot of people. With the ferry, you run the risk of the sea horror, pirates, and numerous other mobs that will kill you, which makes keeping sneak on almost a necessity. Again, making it impractical for anyone without a leveled RDM or WHM job. And that's not mentioning the annoying stops that the ferry captain feels he needs to make.
Another reason you might not see as many nebimonites for sale is just that, in my experience, most people don't like fishing. Unlike the other crafts, you can't take 500k gil into the guild and powerlevel it. It takes a lot of time and patience.
A more cynical view might be that you can't use a fishing bot in either of these places, which keeps the supply lower than, say, moat carp.