Why we use PEP for measuring SSB power?

PEP means Peak Envelope Power.
SSB (Single Side Band) is an AM modulation and requires a linear amplification.
The reason that is used PEP measurement is because all the Power Amplifiers has a limited linear output power, and we have to know where is the limit the amplifier starts to clamp down the signal.
If RMS is used, the output signal could be already limited and the modulation distorted.

vfone is right.
I may add, if you use ssb for voice transmission, the dynamic range of the voice signal is very high. Also the peak value is much higher than the RMS value for voice signal compared to a sine wave, and the values are not precisely defined. The amp should not exceed the peak envelope power specified to obtain a clean signal, without distorsion. You may use a compressor to reduce the dynamic of the voice signal or a clipper to cut the high peaks. This helps to efficiently modulate the transmitter, as you can increase the RMS value keeping the peaks down, to get a distorsion free transmission.