You're looking in the wrong place on this.
There's one correlating factor for both platforms that doesn't show up on the others so much-- ease of piracy.
Both PS1 (and PS2 initially) required semi-expensive and difficult hardware modification that not everyone could do easily. The soldering, especially on PS2, would be very delicate work.
On the other hand, Dreamcast piracy was so easy a 12-year-old could do it without assistance. Download ISO, burn in appropriate tool. Same goes with PSP-- download custom firmware, install it by the instructions, download and install ISO to memstick.
With piracy that easy, most people said, "Why buy something I can get for free?" and stopped buying games on that platform. Nintendo's having some major issues with that on the DS side now as well. It's also very true on PC-- the easier it is to pirate, the more likely people will take the easy and cheap way out.
Try checking the MAME manual at http://docs.mamedev.org