Very good question. It's merely a mystery/conspiracy (WIN!...sorry XP) why most call the PS1 the PSX. When i first tried emulation i found out that the PS1 is also called PSX and never knew why. With what Gare said......she really makes sense =P and that's how i took it when i found out. Humans, if you may already know, have the tendency to make catchy names for loads of electronics. Ex.
Nintendo Systems...
Nintendo Entertainment System: NES
Super Nintendo Entertainment System: SNES
Nintendo 64: N64
and so on...
Those nicknames/acronyms =P were created because it was catchy and easier to say in a sentence. (Can't imagine saying Super Nintendo Entertainment System 5 times in a sentence o.o)
So I guess the PS1 was called the PSX because the name sounded original, catchy and was easier to say in a sentence.
So.... After all that blabbing... What I'm really trying to say is, try to interpret why the name of the PlayStation One is now well known as the PSX on your own terms, because really anything you come up with could have been the reason why to it was given that name. =P
P.S I hope i made sense

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