Curd and sour substances are acidic in nature i.e, they have acids in them. Brass and copper vessels are metals.
When acids react with metals they form salt and hydrogen gas.
So,when the curd or sour substances are kept in brass and copper vessels ,they react with them and can corrode the vessel and form toxic substances which are very harmful for the human body . That's why the curd and sour substances are not kept in brass or copper vessels.