XT, AT, PS/2, and ADB are all serial buses. The key differences with ADB are that it supports both daisy chaining and hot-swapping (the latter is officially proscribed due to lack of surge protection in hardware — but it worked all the same). The IBM PS/2 was idiotic in that it stuck to the outdated notion of dedicated, single-device ports, and lacked a generic high-speed bus, something Acorn offered in 1981 (the 1 MHz Bus), and Apple in 1987 (SCSI), both daisy-chained so you could have more devices than you had ports.
You're probably thinking of the fact that the Mac never had a Centronics port — Macs didn't use parallel printers. Apple also gave you two serial ports so that you had one for the printer and one for a modem.
Ports on the Macintosh were sensible; it's PCs where you had to connect the scanner to the Zip drive, and the joystick to the sound card.
Mac ADB keyboards did have a weird implementation where you had one socket on either side, and each one could be upstream or downstream: you could connect either side to the computer, and the opposite side would become the downstream port (e.g. for the mouse), allowing your keyboard cable to be short and tidy and keep out of the way.