S-parameters are used in microwaves :

• The H, Y, Z and ABCD parameters are difficult at  microwave frequencies due to following reasons.
• Equipment is not readily available to measure total voltage and total current at the ports of the networks. 
• Short circuit and open circuit are difficult to achieve over a wide range of frequencies.
• Presence of active devices makes the circuit unstable for short(or)open circuit. 
• Therefore, microwave circuits are analyses using scattering (or) S-parameters which linearly relate the reflected wave’s amplitude with those of incident waves.