Using S band widely in radar systems?

one reason is S band less attenuated by water molecule in air compared to C or X band
specially in cloudy atmosphere S band will give better results compared to c or x band

there will several other reasons, like amplifier design, electronics design at S band

S band have more power to transmit, if X band transmit same power, it would be very expensive.

And another reason is LO generator's phase noise. S band phase noise can reach -125dBc/Hz@1KHz easily, but X band it would be more difficult.

One reason is that higher frequency signals are prone to attenuation in atmosphere and Radar absorbing material

thanks all that was very helpful