There have always been leased covered hoppers, but they've become much more common in recent years. Partly it's for the same reasons tank cars have long been leased or private owner. Large industries can lease their own cars, and guarantee availability, rather than relying on the railroad having enough cars at the right time. Think grain harvest. Also, as cars, and loads, become more specialized, and cross-contamination becomes more of an issue, it makes sense to control the car, to make sure that no one puts something in it that will affect your next load.
