Well, some of the more expensive black olives come in jars, which leads us to the answer. Black olives are stored in oil, green olives are stored in brine. Brine would eat through a tin can in about a week, so to avoid a really icky mess, green olives get stored in jars. However, jars are by far more expensive than cans, so since black olives are safe in tin cans, they get stored that way. Some black olives are sold in jars for convenience and because it looks better, but at a premium price.