CMOS process provides lower power consumption and is easy to scaling down.
Gate of MOS needs much lower driving current than base current of bipolar.
Scaling down increases CMOS speed.
The most important advantage of CMOS is the very low static power consumption in compare with NMOS technology. On the other hand, CMOS technology is more complex to fabricate then NMOS technology, so it is more expensive. However, almost every todays digital circuits are CMOS. You want to use NMOS only when you want to fabricate fast and low-cost a simple circuit.