Question

why pickles and curd are not stored in copper and brass utensils

Answer 1

When pickles and curd are kept in copper and brass utensils, they react with copper and forms toxic compounds which is harmful for human consumption and may even cause food poisoning. Thus we should not keep pickles and curd in brass and copper utensils.

Answer 2

sour substances are cidic in nature,i.e, they have acids in them. So, when they are kept in brass or copper vessels, they react  with them and form poisonous or better to say toxic elements which may prove harmful for human body. So they are not kept in metal containers.