PS/2 interface has existed since Windows 95 (that I can remember) and, while all another interfaces were upgraded (USB, USB2, USB3, …) or just died (such as Parallel, and the large one used for keyboards), most desktops came with a PS/2 connector until recently. Newer ones, such as USB, HDMI, FireWire, and others are just born being plug-and-play. However, PS/2 follows the same specification from more than 10 years ago (someone correct me if I am wrong) and it doesn't seems to change in a few years.
Today this is the only non-plug-and-play interface that I can remember, so there is some reason for that? What determines if a interface is or isn't plug-and-play?