Based on leaks from Xiaomi insiders, the main reason for the stock shortage of Mi 6 is actually not intentional to create scarcity or open due to slow production process. However the components used in the Mi 6 design are too high and the main components are not supplied in large quantities to meet demand.
In other words there is a major component needed in assembling the Mi 6 which is hard to get. The main component that slows production there is a body part made of ceramic glass. You could say the material for the body of Mi 6 is quite expensive and difficult to produce, so the supply of Mi 6 is limited.