There is a conspiracy theory
that the reduction in parts on the back of the original PSX
systems (I say PSX to distinguish from the compact PSOne
model) was Sony attempting to crack down on the rising problem of piracy
The 7xxx series of PSX, specifically the 7502, was the last model to feature the I/O port on the back. Intended for things like VCD adaptors and cheat cartridges, this was removed for the 9xxx series. This instantly rendered cartridge-based accessories like GameSharks and Action Replays worthless.
This was due to an infamous secondary function of these cartridges, which allowed the playing of copied or imported games by booting the cartridge with an original, correct region CD and then swapping to the copy and selecting 'start game' from the cheat menu. One cartridge, the Game Enhancer (a bootleg Action Replay) was marketed specifically for this use, and included a custom spring designed to hold the door button down even if the drive door itself was open.
The 9xxx series was also significantly different in that it featured a new design of casing and construction which made the dismantling necessary for modification (aka 'chipping') notably more difficult than in its predecessors. Cheat cartridge manufacturers, though, soon made new versions of the Gameshark which worked from CD and the memory card slot, and so it was again possible to play your imports and backups. So much for this attempt at shoring up the so called Playstation copy protection.
Just out of interest, it's my understanding that the 7502 (which was common around the time of the UK drop in pricing to £99 for a PSX) is the most desirable in terms of modification, as in addition to chipping it allows for a quick and easy colour mod. This is a simple attachment of a wire, cutting a track on the mainboard, and as if by magic all games play in full colour regardless of region and system (PAL/NTSC). Yet more proof as to bullshit nature of region encoding and delays for 'PAL conversion'.