Before the PS2 came around, the Playstation was abbreviated as the PSX. The smaller, re-modelled Playstation became the PS1 [PSone] but the original Playstation is still called the PSX. It made no sense to call it the PS1 when there was no Playstation 2.
It's similar to how the Sega Mega Drive has the first version, MD, and the re-modelled version, the MDII. The original Mega Drive is still called the MD, despite it being the same as the MDII in all but appearance.