Why it is impossible to put AC ven in Mahindra for 2nd and 3rd passenger row?

It is usually not impossible, but it is rarely practical to add proper 2nd- and 3rd-row AC vents if the vehicle was not designed for them from the factory. A usable rear AC setup needs more than vents: it needs ducting, blower capacity, evaporator sizing, trim space, and wiring/control integration.