Because these sour substance are acidic in nature(recall that acids are sour,like how alkalis are bitter), they tend to react to the metallic environment, and end up causing harmful byproducts which should not be consumed.

Pickles and sour substances contain organic acids which are weak acids. These acids will react with the metallic part of the vessels to form salts. The salts formed are poisonous and hence dangerous to our health. Therefore, the above substances should not be stored in brass and copper vessels.