'Cause it was the first mouse, and just so damned cool! :)
Notice the DB9 connector - this puppy plugs onto a card slotted in my Apple][+. Also: it only had 9 pins and there's no electronics on the board - the quadrature signals go down the line to the computer (where the circuitry is). With power, ground, button, and 4 detector lines, it only leaves 2 wires. They _could_ have added another 2 buttons; but depended upon using the modifier keys instead - nobody ever _thought_ more than one button was necessary.
