redpill: I like PS/2, but if any-one-of-these-ports works, why dedicate a port? Every living room has a TV, why not have a dedicated power connector in the wall for it?
The thing I dislike about USB is that it doesn't take a lot of force to pull a cable loose. (HDMI isn't much better.) OTOH, it's easier to plug in by feel alone -- PS/2 keyboard connectors were often made in such a way that you could tell where the "top" of the connector was, but anyone who's had to reach behind a desktop computer to plug one in blind knows it's still a fussy operation, and that's before you consider that computer manufacturers sometimes put the ports in "upside-down" on some models.
I still prefer PS/2, but it's because I'm old and nostalgic and my favorite keyboards use it. I know that it's the past. I stopped hearing stories from techs about USB keyboards that would only work in a particular USB port on a machine a couple of years ago (finally retired all the HP Compaq D510 and D530 USDT)... of course that was because they switched back to PS/2, but they've since switched to USB again; seems HP finally figured it out.
And I'm starting to pay for my nostalgia, since PS/2 keyboard ports on new hardware don't really work right anymore, no doubt a side effect of the dwindling numbers of old PS/2 keyboards and the general disinterest in them.
What's the last Apple computer that had a PS/2 port? Where did FireWire go? Why don't printers use parallel ports? Why aren't external hard drives still using SCSI? Where's my game port? What am I going to do with this box of DB9-DB25 serial adapters?
Standards are good, and USB is a mature standard that's suitable for keyboards. PS/2 is hanging around because some big corporate/government purchasers are slow to accept change, and there are still some pockets of infrastructure (old switchboxes, for example) that call for it. But I'd expect the next generation of motherboards from each manufacturer to be PS/2 free, except maybe for one corporate model and one gamer-elite model, and the latter just to cater to the prejudiced.
Any technical benefit PS/2 might have enjoyed is mere trivia in the face of USB's advantages. USB is easier to plug in, it's ubiquitous, and it's hot-pluggable. USB's problems are in the past; PS/2's biggest problems are in its future.
As for why the PS/2 is not still the standard, the above-linked Peeter Seebach article hosted by IBM developerWorks nails it: Apple's USB-only iMac forced the industry to adopt USB. And we're much better off for it.
In fact, with USB 3.0, we could and probably should lose all those audio ports, and even HDMI could get the "legacy" tag. Ethernet too except that it still offers greater distance. Okay, HDMI is well-entrenched, and I can already hear the arguments about how USB isn't stout enough or specialized enough or supported enough, but give it time...
