A pregnant woman not to eat seafood or not too much of it

Why would a doctor tell a pregnant woman not to eat seafood or not too much of it?
What if the pregnant woman loves and has cravings for seafood during her pregnancy? How come seafood
could affect the normal growth of the embryo/fetus in the mother’s womb? Can eating some seafood would be
fine during pregnancy? What is it with fish and/or shrimp, for example, that can connect it to the development
of a human embryo/fetus? How did we get here, to this crazy point where pregnant women (and all peoples, it
seems) need to refrain from eating high quantities of seafood? Isn’t seafood good as a source of amino acids
and also of omega fatty acids for the developing embryo/fetus?