Why substrate in nmos connected to ground and pmos connected to vdd?

In CMOS technology, the NMOS transistor's substrate is connected to ground to prevent parasitic effects and ensure proper operation, as it helps maintain a lower threshold voltage for the NMOS. Conversely, the PMOS substrate is connected to VDD to keep its threshold voltage stable and ensure that the PMOS operates correctly in the enhancement mode. This arrangement minimizes unwanted channel formation and enhances performance by reducing leakage currents in both types of transistors.