>>212843423In kashrut (Jewish dietary law), cookware must be kosher, meaning it can’t absorb and transfer non-kosher food.
Cast iron is porous and can absorb flavors, so:
If it's used with non-kosher food, it needs koshering, which is harder with cast iron.
If it's used for both meat and dairy (not allowed in kosher kitchens), it can cause problems.