Because keyboards back then supported both the HID and legacy PS/2 protocols. the MCU inside the keyboard knew to switch into legacy mode when it detected the PS/2 clock frequency. Optical mice were this way as well, and they too, dont support dual protocols anymore except for more expensive ones.

Some more expensive models still do, but ive noticed the cheapo 10 dollar keyboards that you can pick up at the garden variety store no longer support legacy PS/2 so as soon as you put the little "green" adapter on it, nothing happens.