and more here
https://www.quora.com/Why-heavy-trucks-do-not-use-disc-brakes
Air disc brakes are already being used in trucks mainly in Europe, but their usage in US and India is less. With the large diameter of truck wheels, it is possible to design an effective drum brake with the required large diameter to meet the regulations on stopping distance. The cost of air disc brakes is significantly higher than that of drum brakes of comparable nominal performance.
Though air disc brakes have several advantages, such as consistency in brake performance, less tendency for fading of brakes, and easy changing of brake pads, their application in trucks is limited mainly because of the cost factor.
Incidentally, the use hydraulic disc brakes like the ones used in cars are not possible in truck applications, since the design becomes challenging for the considerably larger weight of the trucks. Moreover, air operated disc brakes have an added advantage when a trailer is attached, since it is easier to connect pressurized air to the trailer for braking through a coupling. Hydraulic connection is less reliable, since even a very small leak in the coupling can result in complete loss of braking. Minor leakage of air in the air-operated system does not cause a major drop in brake performance, because of the availability of reserves in the form of compressed air from the reservoir in the system.