Over exploitation, sacralisation, limited husbandry, elite religious practice?
Why not a sacral tradition that survives until 500BCE? Sacred traditions can excuse stuff that wouldn't publicly be accepted, and tend to have a slower cycle of replacement than secular actions.
If we can get it to 500BCE then the Romans* are going to want to duplicate it just to show off that they can eat everything.
yours,
Sam R.
Might work, but it has to be not obviously anti-economic. Sacred traditions made Egyptians keep crocodiles in captivity for centuries, but did not produce domesticated crocodiles.
Two problems I see are:
1) that snails are relatively finicky; they don't do very well with drought or cold, and investing in them a significant food item requires quite an effort. Probably they are going to be tricky to stockpile and preserve; not impossible, but necessitates specialised techniques that will work for them only. And stockpiling and preservation are pretty important for the significance of central food items.
2) Specialised techniques for snail husbandry will work, as said above, for them only. Most human domesticates fit a specialised niche into a relatively homogenous array of broad techniques that apply to more than one species. You can preserve and prepare meat and leather more or less the same way, regardless it is from ox, sheep, horse or pig. You feed horses or oxen with more or less the same kind of stuff. You treat camel or sheep wool in a broadly similar way.
There are specific techniques that are developed, for example for bees and silkworms. Both are managed with methods that would not work at all for any other domesticate, methods that were developed and refined because both provided something with very high return on investment that could not be obtained otherwise. Wax, honey and silk (let alone other bee byproducts) are high value stuff. Remarkably, bees are not eaten ever and I think neither are silkworms.
Domesticating snail will require developing a lot of specialised snail-related expertise about feeding, breeding, preserving, protecting, cooking etc. them, and none of this is likely to apply to any other domesticate in any straightforward way.