In trucking, every rupee makes a huge difference. The cost of paying extra for fuel spent on ac will prove less competitive. So a driver with Ac might lose out of business to a driver without ac.

Stop and break lights, seat belts, under bars should be made mandatory first right???

Sounds like a good idea on paper. But in reality both the trucks and drivers are overloaded. In the US trucking is regulated with fixed number of hours the driver can drive. In India this isn't the case. So something like AC might actually make drivers more drowsy and cause more accidents.