Why "Rev1" is used in systems like PSX and "v2.00" is used is systems like PSP or PS2?

"Did you ever wonder why we had to run for shelter when the promise of a brave new world unfurled beneath a clear blue sky?"

I'm not a Sony expert but looking at the dumping guides for PS2 and PSP there are files within the image that contain version numbers so those are used.

PSX likely doesn't have files explicitly listing version numbers like the other systems so they use the more generic "Rev" versioning.

Someone more knowledgeable could give you a better answer though.

It is exactly as Agent said.  The version numbers for PS2, for example, are internal to the data, and thus accurate versions for those releases.  Whereas PS1 has no internal version numbers, so identifiers such as Rev 1, Rev 2 are used.