I got curious about this and found a lot of people in older threads saying that farming sardines can't be done. Apparently small schooling fish are typically very difficult to farm because they're pretty fragile and easily stressed.

Whether that's a technical barrier imposed by the biology of sardines being unsuited for how we do fish farming today, or more of a barrier of "it will never make financial sense to do this because so many in each batch would die" I don't know.