CNR Vs. SNR

why C/N is referred for satellite communication instead of S/N ?

I assume that you are asking about Carrier to Noise Ratio, rather than Centre for Nursing Rehabilitation?
It helps if you state the thing in full rather than just as a disembodied TLA.

CNR - Carrier to Noise Ratio is a measure of the received signal strength, where as SNR Signal to Noise Ratio relates to the signal demodulated from the carrier.

Depending on the method of signal recovery, the SNR could be significantly greater or less than the SNR.

JimB