Snails are easy to breed, keeping them alive to sale is a different story.
Actually a lot of animals in this hobby are easy to get spawning, what happens past that point is where the difficulty comes in because in most cases either they're;
A) cannibalistic in their 'larvae stage' (a lot of crustaceans will just eat whatever small organism is floating around with them, especially predatory ones) and separation is very unrealistic/difficult to maintain.
B) incredibly fragile, not just to water quality but easily destroyed by equipment, high flow, etc.
C) opposite of A, extremely picky and not easy to sustain food wise or have a very hard to work around diet swap as they age. (harlequin shrimp come to mind, it was recently achieved a few years ago and sold but it's been radio silent on that breeding project for some time now and haven't seen any in-stock, its unlikely your average hobbyist could achieve that.)
D) randomly flip the switch on what living conditions they need. Usually the case for freshwater/terrestrial invertebrates, your land hermit crabs aren't captive bred outside of maybe one or two individuals that went all out with it, (and this is something that is desperately in need of captive breeding for how many are killed from misinformation and being treated like throw away pets despite being very long living.) Jellyfish also come to mind because they go through several stages so typically you'd only see public aquariums or jellyfish specialist doing breeding projects with them.
When you factor everything in, you then get to the ultimate question; "is this even worth breeding with moderate success compared to catching them?". For threatened species (like clownfish at the time because their population was rapidly declining since... finding nemo.) their payoff for tank raising is not only massively profitable but also prevents a long term issue down the road. Jellyfish are hard to come across but since they're so short lived its more beneficial for public aquariums to breed in-house them than search and pay for replacements they might never find, especially with rare ones, while jellyfish specialist selling them profit off the fact they do need replaced and even if their demographic is niche they rotate their stock fast.
Now trochus snails, speaking purely hypothetically, not only is the payoff kinda niche since not many would specifically search for tank raised ones (maybe what happened to the harlequin shrimp cause their target demographics are niche too) but catching them isn't concerning as their population is pretty stable and you don't have to wait for their growth past the 'getting them to live' stage. The odd rare case where someone manages to have a pair accidentally breed and the babies accidentally survive and grow, usually they decide to keep them cause hey, free CuC for them and its unlikely they'd rear a large sellable quantity, more likely they get 1 or 2.
