Although, some crustaceans have an issue where they harbor bacteria that while helpful to the fish while living, can bloom out of control very quickly when they die. This is why lobster should only be eaten if frozen live or cooked from life. If they die and are allowed to sit, even in refrigeration for a short period of time, they can become problematic to consume.
So maybe it did mean all shellfish from the beginning because they didn’t know exactly what made them sometimes ok to eat and other times not. So better safe than sorry. There is a really extensive and specific list of all the animals off-limits to eat in Leviticus and most of them come with a little tidbit of why usually some physical marker like where its knees are or how its feet look.