I don't get it, why is eating shellfish so bad in the bible? What is it about shellfish?

Ancient peoples tended to ociate things happening with the gods. Living in a great big desert without modern understandings of hygiene and food preparation and without refridgeration, people probably quite often got sick from eating shellfish. People umed this was "god" being angry at them for eating them, so they suggested that god didn't want them to.