>>717958016
Steel Chests go from 48 to 62 to 76 slots between Common and Rare, with Legendary having 120 (this many) slots. The ease of making them is also a factor I think, given that all you need to do is either craft quality Steel or upcycle the chests (if not both).

Only place they dropped the ball is cargo wagons. They don't get any benefits from quality-- not even an hp boost.