Why would anyone use DOT fluid and vacuum-based brakes?
Advantages of DOT over Mineral Oil:


DOT has a standard - product that meets the minimum standards is readily available at low cost
Compatibility with cheap materials used for seals
Hygroscopic, so water will not collect in a single part of the system


Disadvantages of DOT over Mineral Oil: 


Corrosive despite inhibiting additives, damages paint, skin, metals etc.
Hygroscopic, so water is absorbed and the boiling point drops over time. Limited shelf life and operating life.